docs: SEP-2243 clarifications for x-mcp-header

[mikekistler]

Summary

Addresses clarification issues raised during implementation of SEP-2243 (HTTP Standardization), primarily from Go SDK implementers in #2243.

Fixes #2762

Changes

1. RFC 9110 token rule for header names — Replace loose "ASCII excluding space and colon" constraint with RFC 9110 tchar syntax and explicit CR/LF prohibition. Adds link to RFC 9110 Section 5.1.

2. Base64 sentinel collision — Require clients to Base64-encode any plain-ASCII value that matches the =?base64?...?= sentinel pattern, preventing ambiguity.

3. Nested property support — Clarify that x-mcp-header MAY be applied to properties at any nesting depth, not only top-level properties.

4. Transport scoping — Scope x-mcp-header rejection to Streamable HTTP transport only; clients using other transports (e.g., stdio) MAY ignore annotations entirely.

5. Numeric comparison precision — Add implementation note: numeric fields should be compared numerically (within 1E-9 relative precision) rather than as strings.

6. Stale/missing schema retry — Add note on client behavior when tool schema is unavailable: send without custom headers, retry after tools/list if rejected. Also allows pre-loading tool definitions from other sources.

7. Case-sensitive sentinel markers — Clarify that =?base64? and ?= markers are case-sensitive and must appear exactly as shown.

8. Version-gating for intermediaries — Add note that intermediaries enforcing policy on mirrored headers should verify MCP-Protocol-Version indicates a version requiring header–body validation, and reject otherwise.

[tarekgh]

Left minor comment, LGTM otherwise.