Fix localized session equivalents

[Yuxin-Qiao]

Summary

• format session-equivalent plural strings with the locale of the resource bundle selected by L
• keep application-language fallbacks and plural rules aligned
• add regression coverage for resource-locale fallback

Why

When the system locale and the selected or fallback application language differ, the previous formatter could use plural rules from one language with strings from another. For example, English could render 1 windows instead of 1 window.

Validation

• focused Swift test filter for SessionEquivalentForecastTests and LocalizationBundleCacheTests: 43 tests passed
• make check: passed
• full make test: the prior SessionEquivalentForecastTests failure is fixed; the run now reaches group 49 and stops on unrelated Spend Dashboard test isolation failures. Existing test(repo-hygiene): restore macOS test baseline #2418 addresses the Controller subset; remaining Token Provenance coverage will be kept separate.

UI proof

Not applicable: localized text behavior is covered by deterministic unit tests.

Provider / privacy

No provider access, credentials, account data, or network behavior changed.

[clawsweeper]

ClawSweeper status: review started.

I am starting a fresh review of this pull request: Fix localized session equivalents This is item 1/1 in the current shard. Shard 0/1.

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[Yuxin-Qiao]

Superseded by #2418, which now contains this localized plural fix together with the complete macOS test-baseline restoration and full validation.