refactor(search): centralize the web-scraping User-Agent into one constant (#4325)

The outbound UA for web_fetch / web_search was inlined in four places with
two different values and nothing keeping them current: content.py pinned a
mid-2021 Chrome 91 build, and providers.py sent a bare Mozilla/5.0 in three
spots. Some sites serve a degraded or blocked page to a UA that old.

Add WEB_FETCH_USER_AGENT to src/constants.py (env-overridable, matching the
existing Copilot/Kimi UA-constant pattern) and import it in content.py and
providers.py. Default to a current, common desktop UA so pages return their
normal HTML: the market-leading desktop OS (Windows; NT 10.0 covers Windows
10 and 11) and browser (Chrome) on a current stable build. The version is now
bumped in one place.

Service-specific self-identifying agents (Copilot, Kimi, webhooks, cookbook)
are intentionally left separate. Adds a regression pinning the constant shape,
the env override, and a guard against a new inline Mozilla literal in the
search sources.

Closes #4324

services/search/content.py

@@ -15,7 +15,7 @@ from urllib.parse import urljoin, urlparse
 import httpx
 from bs4 import BeautifulSoup
 
-from src.constants import WEB_FETCH_SOFT_MAX_BYTES, WEB_FETCH_HARD_MAX_BYTES
+from src.constants import WEB_FETCH_SOFT_MAX_BYTES, WEB_FETCH_HARD_MAX_BYTES, WEB_FETCH_USER_AGENT
 
 from .analytics import RateLimitError, error_logger
 from .cache import (
@@ -369,7 +369,7 @@ def fetch_webpage_content(url: str, timeout: int = 5, retry_attempt: int = 0,
     # Fetch
     try:
         headers = {
-            "User-Agent": "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/91.0.4472.124 Safari/537.36",
+            "User-Agent": WEB_FETCH_USER_AGENT,
             "Accept": "text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8",
             "Accept-Language": "en-US,en;q=0.5",
             # identity so the streamed size cap in _get_public_url stays honest