fix(cli): skip npm install in protobuf IR generation when packages already installed

[thesandlord]

Description

Refs: Customer report of gRPC generation hanging in air-gapped self-hosted environments.

In ProtobufIRGenerator.setupRemainingProtobufConfig(), two npm install calls (npm install for @bufbuild/protobuf+@bufbuild/protoplugin, and npm install -g fern-api) ran unconditionally. In air-gapped environments where npm cannot reach the registry, these calls hang indefinitely — the root cause of gRPC generation freezing in self-hosted deployments.

This PR uses the existing this.isAirGapped flag as the primary guard to skip all npm installs in air-gapped mode (where the self-hosted Docker image already has fern-api globally and @bufbuild packages at /opt/buf-modules). For non-air-gapped environments, package availability is checked before installing as a secondary optimization.

Additionally, the air-gapped detection timeout (detectAirGappedModeForProtobuf) is reduced from 30s to 10s to minimize startup delay in air-gapped environments.

Requested by: @thesandlord
Link to Devin run: https://app.devin.ai/sessions/a5880378995e43e5bc32210f77dcac83

Changes Made

• Air-gapped primary guard: When this.isAirGapped is true, all npm installs are skipped entirely with a debug log message
• Added isModuleResolvable() — checks if a package exists in the working directory's node_modules via fs.stat (used in non-air-gapped path)
• Added isPackageInstalledGlobally() — checks if a package is globally installed via npm list -g (used in non-air-gapped path)
• In non-air-gapped mode, setupRemainingProtobufConfig skips npm install / npm install -g fern-api if the packages are already present
• Reduced air-gap detection timeout from 30s to 10s in detectAirGappedModeForProtobuf() (utils.ts)
• Updated versions.yml with v3.76.1 changelog entry
• Updated README.md generator (if applicable) — N/A

⚠️ Items for Human Review

1. isAirGapped is only set when deps.length > 0: In generateLocal(), air-gap detection via detectAirGappedModeForProtobuf only runs when there are buf dependencies. If a protobuf spec has no deps but the environment is air-gapped, this.isAirGapped remains undefined and npm install will still be attempted. In practice, gRPC specs should always have deps — but reviewers should confirm this assumption holds.

2. isModuleResolvable on temp directory: In the non-air-gapped path, the check runs against a freshly created temp directory that won't have node_modules, so it will almost always fall through to running npm install. This is acceptable behavior for non-air-gapped environments but means the check is effectively a no-op optimization. The air-gapped guard is the real fix.

3. Docker npm wrapper interaction: The self-hosted Docker image already has an npm wrapper that intercepts npm install calls for @bufbuild packages and copies from /opt/buf-modules/node_modules. If the wrapper were working correctly, the hang shouldn't occur — suggesting the wrapper may be bypassed in some scenarios (PATH ordering, permissions, etc.). This code-level fix provides defense-in-depth.

4. 10s detection timeout: The reduced timeout should be sufficient for a network connectivity check, but very slow or high-latency networks could false-positive as air-gapped. Previously this was 30s.

Testing

• Unit tests added/updated
• Manual testing completed — lint passes (pnpm run check)

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