Fix legacy plugin JSON import validation

[roboomp]

Repro

A minimal legacy plugin entry that imports ./data.json with with { type: "json" } reproduced the failure through loadLegacyPiModule() with bun /tmp/omp-json-import-repro.ts: before the fix, Bun raised BuildMessage: Expected ";" but found ":" because the compat graph hook served the JSON file as JavaScript.

Cause

packages/coding-agent/src/extensibility/plugins/legacy-pi-compat.ts collected every relative Bun.resolveSync() target into the extension rewrite graph. That included .json files, so installExtensionGraphHook() intercepted the JSON module and getLoader() fell back to "js", forcing Bun's per-module parser to parse raw JSON instead of using the native JSON loader.

Fix

• Filter relative extension graph targets through hasSourceModuleExtension() before registering them with the rewrite hook.
• Apply the same source-module filter to extension-local bare dependency entries so non-source assets stay on Bun's native loaders.
• Add a loadLegacyPiModule() regression test for a temp extension importing JSON with with { type: "json" }.
• Add a coding-agent changelog entry for the plugin validation fix.

Verification

bun /tmp/omp-json-import-repro.ts now prints { "value": true }; bun test packages/coding-agent/test/extensibility/legacy-pi-inplace-load.test.ts -t 'leaves JSON import-attribute targets on Bun' passed with 1 test; bun check passed; bun run fix reported no fixes needed before commit. Fixes #4687

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