Migrate the validator client to the Gloas builder API (Gloas builder API 5/5)

[ethDreamer]

Final PR of the Gloas builder API stack:

• sign builder request-auth and preferences (REQUEST_AUTH signing domain,
  web3signer message type)
• add the builder configuration store (builder_definitions.yml) and book
  documentation
• add the builder-preferences service and switch block production to
  POST produceBlockV4 with a BuilderConfig body, threading the
  Eth-Builder-Url header through block publication
• remove the now-unused legacy GET produceBlockV4 client methods

Depends-On: #9806

[ethDreamer]

This pull request is part of a Mergify stack:

#	Pull Request	Link
1	Add Gloas bid selection, block production, and bid gossip processing (Gloas builder API 3/5)	#9805
2	Convert produceBlockV4 to POST and round-trip Eth-Builder-Url (Gloas builder API 4/5)	#9806
3	Migrate the validator client to the Gloas builder API (Gloas builder API 5/5)	#9807	👈

[ethDreamer]

Revision history

#	Type	Changes	Reason	Date
1	initial	cb7346e		2026-08-13 23:38 UTC
2	content	cb7346e → d71aa2c	catch up to Aug 10 spec revisions: builder_pubkeys yml list, drop builders with empty auth_data, POST v4 fork param at call sites	2026-08-13 23:38 UTC
3	content	d71aa2c → f107071	fix clippy: unused import and manual_let_else in builder_store	2026-08-14 00:03 UTC
4	rebase	f107071 → edf5c39 (rebase only)		2026-08-14 18:25 UTC
5	rebase	edf5c39 → 2ec9497 (rebase only)		2026-08-17 16:52 UTC
6	rebase	2ec9497 → d921468 (rebase only)		2026-08-18 17:51 UTC
7	rebase	d921468 → 692d025 (rebase only)		2026-08-19 19:31 UTC
8	content	692d025 → d52d1a0	review response: VC submits preferences in bounded 4096-entry chunks via typed SubmittedBuilderPreferences	2026-08-19 21:57 UTC
9	content	d52d1a0 → 636e480	rebase on unstable: implement sign_request_auth_v1 on the new MockValidatorStore (#9756)	2026-08-20 17:50 UTC
10	rebase	636e480 → 0c7a564 (rebase only)		2026-08-20 21:41 UTC
11	rebase	0c7a564 → 3d54857 (rebase only)		2026-08-20 23:35 UTC
12	rebase	3d54857 → 86623b2 (rebase only)		2026-08-21 14:14 UTC
13	rebase	86623b2 → 6c118a1 (rebase only)		2026-08-24 18:03 UTC
14	rebase	6c118a1 → 5e47ae6 (rebase only)		2026-08-25 17:47 UTC