[test-improver] Improve tests for config package

[github-actions]

Test Improvements: config_core_test.go

File Analyzed

• Test File: internal/config/config_core_test.go
• Package: internal/config
• Lines Added: +51

Improvements Made

Increased Coverage

Added three test cases that exercise previously uncovered branches in validateToolResponseFilters via the TOML LoadFromFile path:

• ✅ TestLoadFromFile_ToolResponseFilter_EmptyToolName — covers the strings.TrimSpace(toolName) == "" guard (line 127–128 of validation.go), triggered by an empty quoted TOML key ("" = ".result").
• ✅ TestLoadFromFile_ToolResponseFilter_InvalidJqExpression — covers the gojq.Parse error path (line 136–137), triggered by a syntactically broken jq expression ("map(").
• ✅ TestLoadFromFile_ToolResponseFilter_WhitespaceOnlyValue — covers the whitespace-only filter guard (line 131–132), triggered by a filter value consisting only of spaces.

Previous Coverage (validateToolResponseFilters): 85.7%
New Coverage (validateToolResponseFilters): 92.9%
Improvement: +7.2%
Overall config package: 95.1% → 95.3% (+0.2%)

Why These Tests Were Missing

The existing TestLoadFromFile_ToolResponseFilters only tested the happy path (valid jq filter). The error paths were tested via validateStandardServerConfig in validation_test.go, but the TOML path (LoadFromFile → validateToolResponseFilters) had no coverage for these branches.

Test Execution

All new tests pass:

=== RUN   TestLoadFromFile_ToolResponseFilter_EmptyToolName
--- PASS: TestLoadFromFile_ToolResponseFilter_EmptyToolName (0.00s)
=== RUN   TestLoadFromFile_ToolResponseFilter_InvalidJqExpression
--- PASS: TestLoadFromFile_ToolResponseFilter_InvalidJqExpression (0.00s)
=== RUN   TestLoadFromFile_ToolResponseFilter_WhitespaceOnlyValue
--- PASS: TestLoadFromFile_ToolResponseFilter_WhitespaceOnlyValue (0.00s)
PASS
ok  	github.com/github/gh-aw-mcpg/internal/config	1.675s	coverage: 95.3% of statements

Note: TestBufferRetryLogic/module_has_no_memory in internal/guard is a pre-existing failure unrelated to these changes (nil pointer dereference in wazero wasm memory).

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Pull request overview

This PR improves the robustness of the internal/config package’s TOML loading tests by adding negative test cases that exercise previously uncovered validation branches for tool_response_filters when configs are loaded via LoadFromFile.

Changes:

• Add a TOML test case that ensures an empty quoted tool name key is rejected.
• Add a TOML test case that ensures syntactically invalid jq expressions are rejected.
• Add a TOML test case that ensures whitespace-only filter values are rejected as empty.

Show a summary per file

File	Description
internal/config/config_core_test.go	Adds new LoadFromFile-path tests covering validateToolResponseFilters error branches for empty tool names, invalid jq, and empty/whitespace filters.

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