Implement dynamic header injection for proxy transports. Fixes modelcontextprotocol#679

Adds a mechanism to dynamically inject and update HTTP headers for ongoing proxy sessions, resolving an issue where headers introduced after the initial handshake (e.g, Mcp-Protocol-Version) were not being forwarded to the server.

The core problem was that the `transportToServer` instance was created once with a static set of headers. This meant that critical, dynamically added headers like `mcp-protocol-version` and `last-event-id` were being dropped in subsequent requests.

The solution implemented here is to wrap the `fetch` function used by the HTTP-based transports (`StreamableHttp` and `SSE`). This custom wrapper merges the latest headers from the client with the headers of each outgoing request from the proxy, ensuring all headers are preserved for the lifetime of the session.

Changes:

In `src/index.ts`:
  - Added `sessionHeaderHolders`, a `Map` to store the most recent set of headers for each active session ID.
  - Added `createCustomFetch`, a new helper function that creates a `fetch` wrapper. This wrapper merges the session headers from `sessionHeaderHolders` with the request-specific headers from the SDK (like `Content-Type`)
  - In `createTransport`, use `createCustomFetch` for both the `StreamableHTTPClientTransport` and `SSEClientTransport`. This injects the dynamic header logic into the transport layer.
  - In `/mcp`, `/sse`, `/message` route handlers, added logic to update the `sessionHeaderHolders` map with the latest headers from every incoming client request.
  - Added cleanup logic to the `onsessionclosed` callback and the `/mcp` DELETE handler to remove session data from `sessionHeaderHolders`.
  - In `getHttpHeaders`, more robust handling of `string[]` and `undefined` values from `req.headers`, satisfying the `"strict": true` TypeScript configuration.

Author: cliffhall