[test] Add tests for config.StdinGatewayConfig.UnmarshalJSON and validateAgainstCustomSchema

[github-actions]

Test Coverage Improvement

Functions Analyzed

Function	Package	Previous Coverage	New Coverage	Δ
StdinGatewayConfig.UnmarshalJSON	internal/config	80.0%	90.0%	+10%
validateAgainstCustomSchema	internal/config	86.2%	93.1%	+6.9%

Why These Functions?

Both functions were identified by combining coverage data with code complexity analysis:

• StdinGatewayConfig.UnmarshalJSON uses an alias-pattern to prevent infinite recursion during JSON decoding. The first return err path (invalid JSON input) was never exercised. Additionally, the agentIDSet / legacyAPIKeySet field-tracking booleans had only partial coverage.

• validateAgainstCustomSchema fetches and compiles a JSON Schema from a URL. The compiler.AddResource(schemaID, ...) call on lines 276–281 — reached only when the schema's $id differs from the fetch URL and $id contains an invalid percent-encoded sequence — was never tested. Similarly, the compiler.Compile failure path (schema with an unresolvable $ref) needed coverage.

Tests Added

StdinGatewayConfig.UnmarshalJSON

• ✅ Invalid JSON input → first return err path (4 sub-cases: plain text, truncated object, array, null byte)
• ✅ agentIDSet / legacyAPIKeySet tracking for all combinations of present/absent fields (7 sub-cases)
• ✅ Scalar field values are decoded correctly alongside the tracking booleans
• ✅ Custom UnmarshalJSON is invoked through the standard json.Unmarshal path via StdinConfig

validateAgainstCustomSchema

• ✅ Schema with invalid percent-encoded $id URL causes compiler.AddResource(schemaID, ...) to fail
• ✅ Schema with an unresolvable $ref causes compiler.Compile to fail
• Both use httptest.NewServer + t.Cleanup cache deletion (consistent with the existing test suite)

Coverage Report

Before:
  config/config_stdin.go:54 (StdinGatewayConfig.UnmarshalJSON): 80.0%
  config/validation.go:228  (validateAgainstCustomSchema):       86.2%
  config package total:                                          97.6%

After:
  config/config_stdin.go:54 (StdinGatewayConfig.UnmarshalJSON): 90.0%
  config/validation.go:228  (validateAgainstCustomSchema):       93.1%
  config package total:                                          97.8%

Test Execution

All 14 buildable packages pass (packages requiring unavailable modules — wazero, otel, mcp-sdk — have pre-existing build failures unrelated to these changes):

ok  github.com/github/gh-aw-mcpg/internal/auth
ok  github.com/github/gh-aw-mcpg/internal/config   1.644s
ok  github.com/github/gh-aw-mcpg/internal/difc
ok  github.com/github/gh-aw-mcpg/internal/envutil
ok  github.com/github/gh-aw-mcpg/internal/httputil
ok  github.com/github/gh-aw-mcpg/internal/logger
ok  github.com/github/gh-aw-mcpg/internal/mcpresult
ok  github.com/github/gh-aw-mcpg/internal/oidc
ok  github.com/github/gh-aw-mcpg/internal/sanitize
ok  github.com/github/gh-aw-mcpg/internal/strutil
ok  github.com/github/gh-aw-mcpg/internal/syncutil
ok  github.com/github/gh-aw-mcpg/internal/sys
ok  github.com/github/gh-aw-mcpg/internal/tty
ok  github.com/github/gh-aw-mcpg/internal/version

---

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[Copilot]

Pull request overview

Adds targeted unit tests in internal/config to increase branch coverage for stdin gateway JSON unmarshaling and custom-schema validation, focusing on previously unexercised error/edge paths.

Changes:

• Add tests for StdinGatewayConfig.UnmarshalJSON invalid-input error return and agentIDSet / legacyAPIKeySet tracking behavior.
• Add tests for validateAgainstCustomSchema error branches: invalid $id (AddResource failure) and schema compile failure.

Show a summary per file

File	Description
internal/config/stdin_gateway_unmarshal_test.go	Adds new unit tests covering additional error/branch paths for stdin gateway unmarshaling and custom schema validation.

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• Files reviewed: 1/1 changed files
• Comments generated: 1