fix(discovery): harden Claude inventory fallbacks

[dgokeeffe]

Summary

• tolerate malformed and partial Unity Catalog pagination during Claude discovery
• fall back to the Anthropic gateway inventory when the catalogue cannot provide a complete answer
• preserve valid Claude models instead of failing the entire discovery result

Why

Claude availability should not disappear because one discovery source is incomplete or malformed. This makes the model inventory resilient while keeping the fallback isolated from broader capability classification.

Stack

PR B of five. Depends on PR A (pr/pi-core-grok). It is opened as a draft until A lands; the isolated review delta is 8fc337e..c8e7114.

Planned dependency order: A → B → C → D and E.

• A: #446 — Pi routing foundation
• B: #447 — this PR
• C: #448 — capability-driven discovery
• D: #449 — Pi OSS streaming
• E: #450 — OpenCode native Responses routing

Validation

• 270 focused discovery tests passed
• malformed and partial pagination cases are covered
• Ruff lint and changed-file formatting passed
• ty check src and git diff --check passed
• the complete integration stack passed 2,358 tests with 37 skips; its three failures reproduce unchanged on main

Non-goals

This PR does not introduce capability-driven discovery or alter Pi/OpenCode provider routing.