Add dependency injection documentation and DI-style dependencies

[jlowin]

FastMCP's dependency injection system was underdocumented—users had questions about how Depends() works and there was nowhere to send them. This PR adds comprehensive documentation and makes the API more consistent.

Documentation: New docs/servers/dependency-injection.mdx page covering:

• How DI works (type annotations vs explicit CurrentContext())
• All built-in dependencies with both DI and function approaches
• Custom dependencies with Depends(), caching, context managers, nesting
• Links to Docket docs for advanced patterns

New dependencies for API consistency:

from fastmcp.dependencies import CurrentRequest, CurrentHeaders, CurrentAccessToken

@mcp.tool
async def my_tool(
    request: Request = CurrentRequest(),      # HTTP request
    headers: dict = CurrentHeaders(),          # Headers (graceful fallback)
    token: AccessToken = CurrentAccessToken(), # Auth token (raises if missing)
) -> str:
    ...

The function equivalents (get_http_request(), get_http_headers(), get_access_token()) remain available for use outside DI context.

[coderabbitai]

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Walkthrough

Adds a new documentation page for dependency injection and consolidates dependency-related docs by removing the Runtime Dependencies section from the old context page. Exposes three HTTP-context dependencies (CurrentRequest, CurrentHeaders, CurrentAccessToken) and re-exports them from the public dependencies module. Introduces progress abstractions (ProgressLike protocol, InMemoryProgress, Progress dependency) to provide per-request progress tracking and selects appropriate progress implementation based on execution context.

Possibly related PRs

• Make pydocket optional and unify DI systems #2835: Modifies the same dependency subsystem and adds HTTP-context helpers and related exports that overlap with CurrentRequest/CurrentHeaders/CurrentAccessToken.
• Bring Docket's dependency injection to FastMCP #2318: Extends the server dependency-injection surface with DI plumbing and utilities that complement the new dependency primitives and progress abstractions.

🚥 Pre-merge checks | ✅ 1 | ❌ 2

❌ Failed checks (1 warning, 1 inconclusive)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 40.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.
Description check	❓ Inconclusive	The description covers the documentation changes, new dependencies, and code examples, but lacks the contributors checklist items (issue reference, testing confirmation, documentation updates acknowledgment) required by the template.	Complete the contributors checklist by specifying the related issue number and confirming manual testing, documentation updates, and development workflow compliance.

✅ Passed checks (1 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately summarizes the two main changes: adding dependency injection documentation and new DI-style dependencies for API consistency.

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[coderabbitai]

Actionable comments posted: 2

🧹 Nitpick comments (2)

docs/servers/dependency-injection.mdx (2)

11-19: Add a short prerequisites section before usage.

The page jumps straight into usage without stating required setup (FastMCP installed, HTTP transport for HTTP deps, tasks extra for background deps).

➕ Suggested insertion

 </Note>
 
+## Prerequisites
+
+- Install FastMCP in your environment.
+- Use an HTTP transport (SSE or Streamable HTTP) if you plan to inject HTTP request data.
+- Install the tasks extra (`fastmcp[tasks]`) if you need background-task dependencies.
+
 ## How Dependency Injection Works

As per coding guidelines.

---

372-396: Add troubleshooting for common DI failures.

Common failures (no HTTP context, unauthenticated token, missing tasks extra) aren’t surfaced. A short troubleshooting section would reduce confusion.

➕ Suggested addition

 For advanced dependency patterns—like `TaskArgument()` for accessing task parameters, or custom `Dependency` subclasses—see the [Docket dependency documentation](https://chrisguidry.github.io/docket/dependencies/).
+
+## Troubleshooting
+
+### "No active HTTP request found."
+You're running outside an HTTP transport. Switch to SSE or Streamable HTTP, or use `CurrentHeaders()` / `get_http_headers()` for a safe fallback.
+
+### "No access token found."
+Ensure authentication is configured and the request is authenticated, or use `get_access_token()` when authentication is optional.
+
+### "FastMCP background tasks require the `tasks` extra."
+Install the tasks extra and restart the server.

As per coding guidelines.

[coderabbitai]

Actionable comments posted: 0

Caution

Some comments are outside the diff and can’t be posted inline due to platform limitations.

⚠️ Outside diff range comments (1)

docs/servers/context.mdx (1)

391-391: Fix broken internal link to HTTP request helpers.

Line 391 references #http-requests, but this heading no longer exists in context.mdx. The HTTP Request section was moved to dependency-injection.mdx. Update the link:

Fix

-For HTTP request access that works regardless of MCP session availability (when using HTTP transports), use the [HTTP request helpers](`#http-requests`) like `get_http_request()` and `get_http_headers()`.
+For HTTP request access that works regardless of MCP session availability (when using HTTP transports), use the [HTTP request helpers](/servers/dependency-injection#http-request) like `get_http_request()` and `get_http_headers()`.