Add enableClientWellKnownLookups

[langleyd]

Adds gating (default on) so a deployment can stop Element Call making client
.well-known requests to the homeserver server_name. Set via the
enableClientWellKnownLookups URL param in widget mode (by the embedder) and
enable_client_well_known_lookups in config.json in SPA mode; resolution is
URL param → config → default on.

• Off: always avoid client .well-known — skips the post-login poll
  (widget.ts widget mode, utils/matrix.ts SPA mode) and the legacy MatrixRTC
  foci fallback.
• On/unset: respect the server — only fall back to .well-known for
  transports on a 404 M_UNRECOGNIZED; any other response is authoritative.

Element web embedded:
Complementary to element-hq/element-web#34084.
If removeelement_call.disable in EW config but keep enable_client_well_known_lookups EC will continue to respect it (as long as it passes in the URL Param).

matrix-org/matrix-js-sdk#5245 can then be added on top and will allow EC to function in EW alongside enable_client_well_known_lookups.

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✅ Johennes
❌ david-langley
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[Johennes]

This looks good to me. I think we should trim the comments down a little though as they're quite verbose. Ideally they shouldn't describe anything that's obvious from a method's or variables name or anything that refers to plumbing of the item being documented in some other part of the code base.