[Repo Assist] refactor(config): extract detailForKeyword to eliminate duplicate guidance strings

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Summary

Addresses the duplicate code pattern reported in #6782.

formatErrorContext in internal/config/validation_schema.go previously duplicated 7 identical addDetail() call pairs across two separate paths:

1. A switch on the terminal keyword from the schema location path (primary path)
2. A series of strings.Contains message-content fallback checks

The added deduplication map prevented double-output at runtime, but updating any guidance string required touching both blocks in sync. Missing one would produce inconsistent messages between the primary and fallback paths.

Fix

• Extract detailForKeyword(keyword string) (string, []string) as the single source of truth for the 7 keyword→guidance text mappings
• Add an addFromKeyword inline closure that calls through to detailForKeyword + addDetail
• The switch body shrinks from 26 lines to 6
• The fallback if blocks retain their message-matching conditions but no longer contain any literal guidance strings — they all delegate to addFromKeyword
• Behaviour is identical

Root Cause

The two-path design (keyword-location switch + message-fallback) is intentional and useful. But the guidance text was copy-pasted rather than shared, creating a maintenance hazard.

Trade-offs

• Adds one small exported-private function (~40 lines). Net change is essentially zero lines.
• detailForKeyword is not exported, so it doesn't add API surface.

Closes #6782

Test Status

⚠️ Infrastructure limitation: The Go module proxy (proxy.golang.org) is blocked by the firewall in this CI environment, preventing go build and go test from running. This is a known limitation affecting all recent Repo Assist PRs in this repo.

Manual verification:

• gofmt -d internal/config/validation_schema.go → no diff (no formatting issues)
• Code compiles syntactically (gofmt parses and reformats without errors)
• All 7 error categories are still handled identically — only the indirection layer changes
• The existing tests in validation_schema_error_format_test.go cover both the keyword-location path and the message-fallback path and should still pass

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Pull request overview

This PR refactors JSON schema validation error formatting to centralize keyword-specific guidance text, reducing duplication between schema-location classification and message-based fallback handling.

Changes:

• Adds detailForKeyword as the single source of truth for validation guidance strings.
• Updates formatErrorContext to reuse the helper for both keyword and message fallback paths.
• Preserves existing classification behavior while simplifying duplicated addDetail calls.

Show a summary per file

File	Description
internal/config/validation_schema.go	Centralizes validation error detail strings and delegates both primary and fallback paths through the shared helper.

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• Files reviewed: 1/1 changed files
• Comments generated: 0