Fix async LinkContentFetcher custom client headers

Describe the bug

LinkContentFetcher applies headers configured through client_kwargs to
synchronous requests but not to asynchronous ones.

_get_headers() reads self._client.headers. run_async() initializes only
self._async_client, and _fetch_async() then calls _get_headers(). As a
result, default headers supplied through client_kwargs={"headers": ...} are
included by run() but are omitted by run_async().

Error message

Haystack does not raise a specific exception. A server that requires the custom
header commonly returns an HTTP error, such as 401 Unauthorized or 403 Forbidden, from run_async() while run() succeeds with the same component
configuration.

Expected behavior

run() and run_async() should send the same configured client-default
headers.

Header precedence should remain unchanged:

1. Client defaults
2. Haystack defaults
3. request_headers
4. Rotating User-Agent

Additional context

Affected source: haystack/components/fetchers/link_content.py.

This affects asynchronous pipelines that pass API keys, tenant headers, or
other default headers through client_kwargs={"headers": ...}. The synchronous
path handles those headers because self._client exists; the asynchronous path
does not, because only self._async_client exists.

A likely fix is to pass the active HTTP client to the header-construction helper
or to centralize configured default headers. A regression test should assert
header parity between run() and run_async().

To Reproduce

1. Start an HTTP test server that rejects requests without X-Test-Token.

2. Configure the fetcher:

   fetcher = LinkContentFetcher(
       client_kwargs={"headers": {"X-Test-Token": "token"}},
   )
   

3. Call fetcher.run(urls=[server_url]) and verify that the server receives
   X-Test-Token: token.

4. Create a fresh fetcher with the same configuration and call:

   result = await fetcher.run_async(urls=[server_url])
   

5. Observe that the async request omits X-Test-Token, causing the test server
   to reject the request.

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System

• OS: Windows 11
• Haystack version: current checkout (commit b717d00)