fix(editor): prevent false language mode recommendations

[bajrangCoder]

• recommend only language-mode plugins available in the registry
• suppress built-in, extensionless, and arbitrary file false positives
• remove the direct plugin-request issue action
• add regression tests for unknown extensions

[greptile-apps]

Greptile Summary

The PR narrows language-mode recommendations to unknown extensions with matching registry plugins and removes the plugin-request fallback.

• Filters built-in, extensionless, and arbitrary-file false positives.
• Evicts failed registry lookups so later recommendations can retry.
• Adds sanity and unit coverage for eligibility, registry matching, and transient failures.

Confidence Score: 5/5

The PR appears safe to merge.

No blocking failure remains.

Important Files Changed

Filename	Overview
src/lib/languageModeRecommendations.js	Restricts recommendation eligibility, requires a registry match, removes plugin-request notifications, and correctly evicts transiently failed availability lookups.
tests/unit/languageModeRecommendations.test.js	Adds focused coverage for arbitrary extensions, matching registry plugins, and retries after network or server failures.
src/test/sanity.tests.js	Adds lightweight regression checks for extensionless filenames, built-in modes, and unknown recommendation candidates.

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
    A[Editor file opened] --> B{Unknown extension?}
    B -->|No| C[Do not recommend]
    B -->|Yes| D[Query plugin registry]
    D -->|Matching plugin| E[Show extension recommendation]
    D -->|No match| C
    D -->|Transient failure| F[Evict failed cache entry]
    F --> G[Allow later retry]

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_{Reviews (2): Last reviewed commit: "fix the stale network cache things" | Re-trigger Greptile}

[bajrangCoder]

@greptile