Pin declared Anthropic MPS targets by family, like Bedrock

[masonc08]

Problem

ucode claude --provider <mps> sends whatever model string Claude Code's alias resolves to, and the gateway's direct-MPS path exact-matches that against the service's declared targets, then forwards it verbatim. For a Bedrock service ucode already pins ANTHROPIC_DEFAULT_*_MODEL to the service's target slugs — but for an API-key Anthropic service it pinned nothing and trusted Claude Code's canonical names to match the declared targets.

They often don't: a custom or dated target name, or an enterprise managed-settings pin like ANTHROPIC_DEFAULT_HAIKU_MODEL=system.ai.claude-haiku-4-5, makes the client send a name that isn't a declared target → gateway 403 … not in the allowed models list (and it wouldn't be a valid Anthropic model on egress either).

Fix

Extend resolve_provider_models to derive the {family: target} pin map for a non-relayed Anthropic service too — exactly as it already does for Bedrock. map_claude_family_models already handles canonical Anthropic ids (and yields nothing for non-Claude services), so the client now sends exactly the ids the MPS declares. Bedrock is unchanged; relayed Claude Max/Enterprise stays exempt (the gateway disables model selection server-side for that tier).

The managed (admin-manifest) path already pins Anthropic from authored slots, so this only closes the gap on the developer-configured path.

Caveat — managed-settings precedence

ucode writes ANTHROPIC_DEFAULT_*_MODEL into its --settings file; enterprise /etc/claude-code/managed-settings.json may outrank it. This fix reuses the exact mechanism Bedrock uses, so it takes effect wherever Bedrock MPS pinning already does. Decisive check: does a Bedrock MPS session pick up ucode's pinned slug on an Isaac-managed laptop today? If yes, this is sufficient everywhere; if no, both Bedrock and Anthropic need the separate managed-settings-scoping fix.

Testing

• tests/test_agents_init.py: flipped test_anthropic_returns_no_models → test_anthropic_pins_family_targets; added test_anthropic_with_no_claude_targets_pins_nothing.
• 81 passed, ruff check clean. Bedrock and relayed behavior unchanged (existing tests green).

This pull request and its description were written by Isaac.