Phase 2 of [INF-75](https://linear.app/chatwoot/issue/INF-75). Follows
up on [#14753](https://github.com/chatwoot/chatwoot/pull/14753) (Phase 1
UA pattern fallback, already merged).
When the request carries an \`X-Chatwoot-Client-Name\` header,
\`UserSessionTrackingService\` now prefers the five structured
\`X-Chatwoot-*\` headers over the User-Agent for populating the
\`user_sessions\` row:
| Header | Mapped column |
|---|---|
| \`X-Chatwoot-Client-Name\` | \`browser_name\` |
| \`X-Chatwoot-Client-Version\` | \`browser_version\` |
| \`X-Chatwoot-Device-Model\` | \`platform_name\` |
| \`X-Chatwoot-Platform-Version\` | \`platform_version\` |
| \`X-Chatwoot-Platform\` (+ model) | \`device_name\` (\`iPhone\` /
\`iPad\` / \`Android\`) |
When the header is absent or blank, the existing \`Browser.new\` path
runs, with the Phase 1 legacy UA fallback still acting as the floor.
Real browsers are untouched.
A follow-up PR in
[chatwoot-mobile-app](https://github.com/chatwoot/chatwoot-mobile-app)
will start sending these headers from the React Native build. Until that
ships, this PR is a no-op for production traffic, so it can land
independently.
Fixes INF-75.
