[test] Add tests for config.formatErrorContext, formatValidationErrorRecursive, formatSchemaError (#1435)

## Test Coverage Improvement: Schema Error Formatting Functions

### Functions Analyzed

- **Package**: `internal/config`
- **Functions**:
- `formatErrorContext` — provides human-friendly diagnostic context for
JSON schema validation errors based on message content (7 if-branches)
- `formatValidationErrorRecursive` — recursively formats schema errors
with depth-based indentation
- `formatSchemaError` — entry point that dispatches to the above and
appends version + documentation footer
- **Previous Coverage**: ~0% direct coverage (only reached indirectly
via `TestEnhancedErrorMessages` which requires network-fetched schemas)
- **Complexity**: High — `formatErrorContext` has 7 independent
if-branches plus a keyword-location check;
`formatValidationErrorRecursive` is recursive with depth tracking

### Why These Functions?

`formatErrorContext` has the highest branch density in
`validation_schema.go` with 7 independent conditional blocks, none
directly tested. The existing `TestEnhancedErrorMessages` calls
`validateJSONSchema` which fetches a schema from GitHub over the
network, making those tests environment-dependent and leaving the
formatting functions without reliable direct coverage.

### Tests Added

- ✅ **`TestFormatErrorContext`** (20 table-driven cases):
- All 7 branch conditions: `additionalProperties`, type mismatch
(`expected`+`but got`/`type`), enum (`value must be one of` / `must
be`), missing required (`missing properties` / `required`), pattern
(`does not match pattern` / `pattern`), min/max (`must be >=` / `must be
<=` / `minimum` / `maximum`), oneOf (`doesn't validate with any of` /
`oneOf`)
  - Keyword location shown when different from instance location
  - Keyword location suppressed when same as instance location or empty
  - Prefix string prepended correctly to all output lines
  - Unrecognized messages return empty string

- ✅ **`TestFormatValidationErrorRecursive`** (7 sub-tests):
  - Root-level error formatting with location and message
  - Empty `InstanceLocation` falls back to `(root)` placeholder
  - Depth 0 appends trailing newline; depth > 0 does not
  - Recursive processing of `Causes` slice
  - Nested causes get 2× indentation per depth level
  - Multiple root-level causes are all included
  - Context details from `formatErrorContext` are included in output

- ✅ **`TestFormatSchemaError`** (6 sub-tests):
  - `nil` input returns `nil`
- `*jsonschema.ValidationError` gets full formatted message with
version, location, and documentation footer
  - Non-schema `errors.New` gets compact one-line format with version
  - `fmt.Errorf` wrapped errors get compact format
  - `additionalProperties` errors include context details in full path
  - Recursive causes are included in output

### Test Execution

Tests are designed to pass in CI where Go module dependencies are
available. They use `jsonschema.ValidationError` struct directly (no
network required), `version.Set()` for deterministic version assertions,
and follow the project's table-driven test conventions with
`testify/assert`.

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Author: lpcox