fix: resolve all waiters for plugin loads

[mikamikasuki]

Fixes #2936

waitForPlugin() stored one resolver per plugin ID. When multiple callers waited for the same plugin, later calls replaced earlier resolvers and left those promises pending.

This change keeps all pending callers for each plugin ID, resolves them when that plugin loads, and rejects them all with the existing plugin-specific error when initial plugin loading completes. The already-loaded fast path remains unchanged.

Regression coverage verifies that multiple callers resolve together and that all waiters receive the correct error when loading fails.

Validation:

• vitest run tests/unit/pluginWaiters.test.js
• biome check src/lib/acode.js src/lib/pluginWaiters.js tests/unit/pluginWaiters.test.js
• git diff --check

[greptile-apps]

Confidence Score: 5/5

[Medium risk] Refactors plugin load waiting mechanism.

This PR appears safe to merge; no actionable issues were found.

What we checked:

• Earlier callers stay waiting: waitFor() adds each caller to a shared Set. resolve() settles every caller in that set instead of only the latest caller.

Summary

This PR fixes waitForPlugin() losing earlier callers when several callers wait for the same plugin.

• PluginWaiters keeps every pending caller for each plugin ID.
• A successful load resolves that plugin’s callers. Load completion rejects remaining callers with the existing plugin-specific message.
• New tests cover multiple callers resolving together and rejection across multiple plugin IDs.

Tests were inspected, not run.

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