fix: normalize Anthropic header keys to lowercase in provider registry

[prakersh]

Summary

• Normalize all Anthropic-specific header keys in providerRegistry.ts from PascalCase ("Anthropic-Version", "Anthropic-Beta", etc.) to lowercase ("anthropic-version", "anthropic-beta", etc.)
• Update test assertions to match the new lowercase keys

Problem

The provider registry uses PascalCase header keys (e.g. "Anthropic-Version") while the Claude Code client path in base.ts (added in v3.6.9) uses lowercase keys ("anthropic-version"). Since JavaScript object keys are case-sensitive, both keys coexist in the headers object. When fetch() sends them, HTTP treats them as duplicates and concatenates the values:

anthropic-version: "2023-06-01, 2023-06-01"

This causes Anthropic's API to reject the request with a 400 error:

{"error":{"message":"[400]: anthropic-version: \"2023-06-01, 2023-06-01\" is not a valid version","type":"invalid_request_error","code":"bad_request"}}

Root cause

Two independent code paths setting the same HTTP header with different JS key casing:

1. providerRegistry.ts (v1.0.0) — "Anthropic-Version": ANTHROPIC_VERSION_HEADER (PascalCase)
2. base.ts ccHeaders (v3.6.9) — "anthropic-version": "2023-06-01" (lowercase)

Test plan

• t20-t22-provider-headers.test.ts — passes with lowercase assertions
• search-provider-validation.test.ts — passes with lowercase assertion
• Full test suite (3238 tests) — all pass

[gemini-code-assist]

Code Review

This pull request normalizes several HTTP header keys to lowercase across various provider configurations in providerRegistry.ts and updates the corresponding unit tests. This change affects headers such as anthropic-version, anthropic-beta, and x-app. Feedback suggests further lowercasing remaining headers like User-Agent and X-Stainless-* to maintain consistency and prevent potential duplicate header issues during fetch calls.

[gemini-code-assist]

The normalization of x-app to lowercase is consistent with the Anthropic headers, but the X-Stainless-* headers (lines 281-289) and the User-Agent header (line 279) remain in PascalCase. Since JavaScript object keys are case-sensitive, any other code path setting these headers in lowercase will cause fetch() to send duplicate headers (concatenated values), which is the root cause of the issue described in this PR. Consider lowercasing all header keys in this block for consistency, especially since other providers like github already use lowercase keys (e.g., user-agent).