Keep tenant default from rewriting every agent Model column (CL-6782)

[TheGreatAxios]

Stacked on CL-6748 (PR #602). Changing Settings tenant default must leave existing agents stored Model values unchanged. New agents may still pick up the default at create.

Connections chooseModel PATCHes offering priority only — never agent-definitions. The roster reads stored capabilities.model; unset stays a stable Default, never the live catalog winner. Catalog mapping for a set model still uses displayName from 6748.

Fixes CL-6782

[TheGreatAxios]

Self-review

Verdict: the diff is comments/copy/tests only — the underlying behavior (unset model stays "Default", chooseModel only PATCHes catalog/offerings, never agent-definitions) was already implemented in the stacked CL-6748 PR. No logic changes were needed or made; this PR documents and locks that contract in tests.

• Comment on chooseModel explaining the "offering-priority write only, never PATCH agent-definitions" contract: DISMISSED — explains why, not what, and matches the actual code path (updateOwnOffering only).
• New agents-page.test.tsx cases for unset-stays-Default and stored-model-unchanged across catalog reorderings: DISMISSED — meaningful red/green coverage of the CL-6782 contract, not duplicative of existing CL-6748 cases.
• New connections-default-model-pick.test.tsx case asserting all non-GET fetch calls hit catalog/offerings/* and none hit agent-definitions: DISMISSED — directly encodes the fix's guarantee, not coverage theater.
• Checked applyPatches' .catch(() => onError(...)) for reportError compliance: DISMISSED — pre-existing catch, not touched by this diff's changed lines, out of scope per report-error baseline rules.
• No as T casts, no fallbacks/legacy paths, no secrets, no exactOptionalPropertyTypes hazards in the diff.

No fixes made; nothing confirmed-open.

Targeted tests:

• bun test apps/web/test/agents-page.test.tsx — 33 pass, 0 fail
• bun test packages/settings-ui/test/connections-default-model-pick.test.tsx — 2 pass, 0 fail
• bun run typecheck — clean

[linear-code]

CL-6782