feat: Add dynamic URL support to ClientBuilder

[beekld]

Summary

Adds a way for callers to update the request URL between reconnects,
needed so FDv2 streaming can refresh the basis query param on each
reconnection.

• New ClientBuilder::dynamic_url(watch::Receiver<Uri>). Client reads
  the current value at each connection attempt. If unset, uses the
  existing url field.
• Moves redirect state out of struct fields and into State variants,
  to avoid quasi-global state between the URL source and the redirect
  flow. Semantics-preserving.

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Note

Medium Risk
Touches core connect/reconnect and HTTP redirect logic; behavior is intended to be equivalent but URL resolution timing changed for redirects vs dynamic URLs.

Overview
Adds ClientBuilder::dynamic_url(watch::Receiver<Uri>) so callers can change the SSE endpoint on every connect and reconnect (e.g. refresh query params like basis). When set, resolve_url() reads the current watch value and overrides the static for_url value; tokio gains the sync feature for the watch channel.

Reconnect / redirect refactor: ReconnectingRequest no longer keeps current_url or a global redirect counter on the struct. Each New connect uses resolve_url(); redirect depth lives in State::Connecting / FollowingRedirect, and redirects issue send_request to the Location URI directly instead of mutating a cached URL and re-entering New. Redirect limit behavior is preserved with tests for 301 follow, limit exceeded, and redirect vs dynamic URL precedence.

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