refactor: unify header construction in sendLogs and sendOnClose

[brucearctor]

Summary

sendLogs() and sendOnClose() in sendLogs.ts construct HTTP headers independently with diverging logic. This causes bugs and makes adding new headers error-prone.

Current Divergence

Behavior	sendLogs()	sendOnClose()
Content-Type	✅	✅
Authorization (string)	✅	✅
Authorization (function callback)	✅ calls the function	❌ calls .toString() → sends function source code
x-api-key (#124)	✅	✅
config.headers (custom)	✅	❌ missing
updateAuthHeader()	✅	❌ not called
updateCustomHeaders()	✅	❌ not called

Three bugs from this divergence

1. Function-type authHeader broken in sendOnClose — if authHeader is a callback (e.g., for token refresh), sendOnClose calls .toString() which sends the function source code as the Authorization header value
2. Custom headers missing from pagehide flush — headers registered via registerHeadersCallback() are never applied to the final sendOnClose flush
3. Future headers must be added in two places — every new header type (like apiKey in feat(auth): add support for api keys #124) requires changes to both functions

Proposed Fix

Extract a shared buildHeaders(config): Headers helper:

function buildHeaders(config: Configuration): Headers {
  const headers = new Headers({ "Content-Type": "application/json;charset=UTF-8" });

  updateAuthHeader(config);
  if (config.authHeader) {
    const value = typeof config.authHeader === "function" ? config.authHeader() : config.authHeader;
    headers.set("Authorization", value);
  }

  if (config.apiKey) {
    headers.set("x-api-key", config.apiKey);
  }

  updateCustomHeaders(config);
  if (config.headers) {
    for (const [header, value] of Object.entries(config.headers)) {
      headers.set(header, value);
    }
  }

  return headers;
}

Both sendLogs() and sendOnClose() call buildHeaders(config).

Testing

Add tests for sendOnClose that verify:

• x-api-key header is present (test exists after feat(auth): add support for api keys #124)
• Authorization header works with function-type authHeader
• Custom headers from registerHeadersCallback() are included
• Headers from config.headers are included

Context

Pre-existing issue, exposed during review of #124. The sendOnClose path is the pagehide event handler — it fires when users close tabs or navigate away. Logs sent via this path are currently missing custom headers and have broken auth callback support.