Implement SEP-2243: server-side custom header validation #979

Part of #977.

SEP-2243 (HTTP Standardization) server-side support. The http-custom-header-server-validation scenario fails 0/5 on the 2026-07-28 suite (0.2.0-alpha.9):

• sep-2243-server-no-xmcp-tool — the conformance server exposes no tool with x-mcp-header annotations, which makes the remaining checks untestable
• sep-2243-server-decode-base64 — decode base64-encoded header values
• sep-2243-server-validate-param-match — validate that header values match the designated tool parameters
• sep-2243-server-reject-invalid-param-chars — reject invalid characters in header params
• sep-2243-server-reject-param-mismatch — reject mismatched header/param values

Work: SDK support for x-mcp-header tool annotations on the server side, plus a test tool in conformance/src/bin/server.rs.

Repro:

cargo build -p mcp-conformance
STATELESS=1 PORT=8002 ./target/debug/conformance-server &
npx -y @modelcontextprotocol/conformance@0.2.0-alpha.9 server \
  --url http://127.0.0.1:8002/mcp \
  --scenario http-custom-header-server-validation -o results