feat(models): add Claude Sonnet 5 support

[parashuchandra]

Summary

This PR bundles two related fixes needed to get Claude Sonnet 5 working
end-to-end with Claude Code CLI 2.x through the gateway:

1. Register claude-sonnet-5 as a known model
2. Fix a request-validation bug that blocks Claude Code CLI 2.x from
   working with any model (not just Sonnet 5) — found while testing not work with cursor #1

1. Claude Sonnet 5 support

• kiro/config.py: add claude-sonnet-5 to FALLBACK_MODELS, so it's
  recognized as a known model during DNS-failure recovery instead of an
  unverified passthrough.
• kiro/model_resolver.py: fix a gap in normalize_model_name's
  no-minor-version pattern. Models without a minor segment (claude-sonnet-4,
  claude-sonnet-5) only stripped an 8-digit date suffix, not a latest or
  trailing-numeric suffix — unlike the with-minor-version pattern, which
  already handled all three. This meant claude-sonnet-5-latest failed to
  normalize and incorrectly resolved as unverified passthrough.
• README.md: list Claude Sonnet 5 in Available Models.
• tests/unit/test_config.py: new TestClaudeSonnet5Support — fallback
  registration, name normalization (plain/date/latest suffixes), cache
  resolution (source=cache, is_verified=True), and family-isolated
  suggestions.

2. Accept role=system inline in AnthropicMessage

Claude Code CLI 2.x sends messages with role="system" inline in the
messages array (e.g. injected <system-reminder> context), rather than
exclusively via the top-level system field. AnthropicMessage.role only
accepted user/assistant, so these requests failed with a 422
literal_error — before ever reaching normalize_message_roles() in
converters_core.py, which already exists to convert unknown roles to
user downstream (same lineage as the alternating-roles fix in #64).

Reproduced directly: running claude -p "..." against the gateway failed
with:

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[cla-bot]

Thank you for your pull request and welcome to our community. We could not parse the GitHub identity of the following contributors: parashuchandra.
This is most likely caused by a git client misconfiguration; please make sure to:

1. check if your git client is configured with an email to sign commits git config --list | grep email
2. If not, set it up using git config --global user.email email@example.com
3. Make sure that the git commit email is configured in your GitHub account settings, see https://github.com/settings/emails

[ankitcharolia]

@parashuchandra could you give a try to this gateway: https://github.com/ankitcharolia/kiro-gateway and all models are automatically added , no model needed to be updated

It works quite well with All AI harness and actively being developed. The most important thing is that it is ACP compliant