Improve test coverage to ~100% for @azure/core-client

[deyaaeldeen]

Changes

New test files

• base64.spec.ts (node + browser)
• interfaceHelpers.spec.ts
• operationHelpers.spec.ts
• pipeline.spec.ts
• utils.spec.ts
• urlHelpers.spec.ts (expanded)

Expanded existing tests

• deserializationPolicy.spec.ts: error paths, XML parsing, header deserialization
• serializationPolicy.spec.ts: body serialization edge cases, stream bodies
• serializer.spec.ts: polymorphism, flattening, readonly, date/bytearray edge cases
• serviceClient.spec.ts: auth, custom pipelines, error handling, base URI
• authorizeRequestOnClaimChallenge.spec.ts: challenge parsing edge cases
• authorizeRequestOnTenantChallenge.spec.ts: multi-call token flows

Test quality improvements

• Replaced mutable request capture with vi.fn() pattern
• Replaced try/catch + assert.fail with rejects.toThrow() / toMatchObject()
• Replaced calledOnce flags with vi.fn().mockImplementationOnce()
• deepEqual on primitives changed to strictEqual
• assert.equal(x, undefined) changed to assert.isUndefined(x)

[Copilot]

Pull request overview

This PR significantly expands the automated test suite for @azure/core-client to drive coverage closer to 100%, mainly by adding targeted unit tests for previously untested branches and improving existing assertions/mocking patterns.

Changes:

• Added multiple new internal/public test files to cover core-client utilities, pipeline creation, URL helpers, Base64 helpers, and operation/interface helpers.
• Expanded existing tests for serialization/deserialization policies and the serializer to cover more edge cases (XML/JSON parsing paths, polymorphism, constraints, stream bodies, etc.).
• Refactored some tests to use more robust Vitest patterns (vi.fn().mockImplementationOnce, rejects.toThrow, strictEqual, etc.).

Reviewed changes

Copilot reviewed 15 out of 15 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
sdk/core/core-client/test/utils/serviceClient.ts	Simplifies request capture in shared ServiceClient test helper.
sdk/core/core-client/test/public/serializer.spec.ts	Large expansion of serializer coverage (base64url, date/stream/enum/sequence/dictionary/composite/xml branches).
sdk/core/core-client/test/public/authorizeRequestOnTenantChallenge.spec.ts	Improves mocking of next with vi.fn().mockImplementationOnce for multi-call flows.
sdk/core/core-client/test/internal/utils.spec.ts	Adds tests for flattenResponse paging + stream response shaping.
sdk/core/core-client/test/internal/urlHelpers.spec.ts	Adds tests for additional appendQueryParams and appendPath edge cases.
sdk/core/core-client/test/internal/serviceClient.spec.ts	Refactors several tests toward expect(...).toThrow / rejects patterns and tighter assertions.
sdk/core/core-client/test/internal/serializationPolicy.spec.ts	Adds broader serialization-policy coverage (form data, XML handling, stream handling, custom headers, error paths).
sdk/core/core-client/test/internal/pipeline.spec.ts	Adds tests for createClientPipeline behavior with/without credentials and default options.
sdk/core/core-client/test/internal/operationHelpers.spec.ts	Adds tests for composite parameter paths and originalRequest symbol behavior.
sdk/core/core-client/test/internal/interfaceHelpers.spec.ts	Adds coverage for getPathStringFromParameter fallback behavior.
sdk/core/core-client/test/internal/deserializationPolicy.spec.ts	Adds many new deserialization-policy tests for parsing errors, XML branches, header mappers, and response selection.
sdk/core/core-client/test/internal/base64.spec.ts	Adds base64 helper test for Uint8Array.
sdk/core/core-client/test/internal/node/base64.spec.ts	Adds base64 helper test for Node Buffer input (node-only).
sdk/core/core-client/test/internal/authorizeRequestOnClaimChallenge.spec.ts	Expands CAE/WWW-Authenticate parsing edge cases and reduces request creation duplication.
sdk/core/core-client-rest/test/internal/clientHelpers.spec.ts	Adds regression test to avoid mis-detecting non-string key as KeyCredential.

Comments suppressed due to low confidence (2)

sdk/core/core-client/test/internal/serviceClient.spec.ts:101

• request is declared and only assigned within the sendRequest callback, but never read. With noUnusedLocals: true this will fail TypeScript compilation. Remove the variable (and any related capture) from this test since the constructor is expected to throw before any request is sent.

      expect(() => {
        let request: OperationRequest;
        new ServiceClient({
          httpClient: {
            sendRequest: (req) => {
              request = req;
              return Promise.resolve({ request, status: 200, headers: createHttpHeaders() });
            },

sdk/core/core-client/test/internal/serializationPolicy.spec.ts:255

• There are two consecutive test cases with the identical title "should serialize a JSON Stream request body". This makes it hard to identify which case failed when a test breaks. Rename one of them to reflect the specific behavior under test (e.g., the xmlNamespace-related case).

    it("should serialize a JSON Stream request body", () => {
      const httpRequest = defaultRequest();
      serializeRequestBody(
        httpRequest,
        {
          bodyArg: "body value",
        },
        {
          httpMethod: "POST",
          requestBody: {
            parameterPath: "bodyArg",
            mapper: {
              required: true,
              serializedName: "bodyArg",
              type: {
                name: MapperTypeNames.Stream,
              },
            },
          },
          responses: { 200: {} },
          serializer: createSerializer(),
        },
        stringifyXML,
      );
      assert.strictEqual(httpRequest.body, "body value");
    });

    it("should serialize a JSON Stream request body", () => {
      const httpRequest = defaultRequest();
      serializeRequestBody(
        httpRequest,
        {
          bodyArg: "body value",
        },