feat(mcp): add structured error categories for agentic error handling

[sneharathod7]

# Changes

- :gift: Add structured `ErrorCategory` support for MCP tool failures
- :broom: Implement intelligent CLI error classification with remediation hints
- :broom: Resolve relative executable path issues using `os.Executable()`
- :broom: Update MCP `build` and `deploy` handlers to return structured error responses

/kind enhancement

Fixes #3750
Relates to #3752

---

## Summary

This PR introduces structured error handling for MCP tools to improve agent decision-making and remediation workflows.

Currently, MCP handlers primarily surface failures as raw CLI strings, making it difficult for agents to reliably determine whether a failure originated from:
- registry authentication
- Kubernetes connectivity
- build/runtime failures
- invalid local configuration

This change adds machine-readable error categories along with actionable remediation hints so agents can respond more intelligently instead of relying on fragile string parsing.

This work also complements the `check_prerequisites` MCP tool by ensuring that operational failures return high-confidence failure signals during runtime workflows.

---

## What This PR Adds

### Structured Error Responses

MCP tool failures now return structured JSON responses:

```json id="w6zcl0"
{
  "errorCategory": "REGISTRY_ERROR",
  "message": "unauthorized: authentication required",
  "hint": "Check your registry credentials. Try running 'docker login <registry>' first."
}

---

Error Categories Implemented

• REGISTRY_ERROR
• CLUSTER_ERROR
• BUILD_ERROR
• VALIDATION_ERROR
• AUTH_ERROR
• UNKNOWN_ERROR

---

Intelligent Error Classification

Introduced heuristic-based classification for common CLI failure patterns across:

• registry operations
• Kubernetes deployment flows
• build failures
• authentication issues

This allows MCP agents to provide targeted remediation guidance programmatically.

---

Absolute Path Execution

Updated MCP command execution to resolve the running binary path using:

os.Executable()

This fixes issues such as:

exec: "func": cannot run executable found relative to current directory

which can occur in newer Go versions and certain shell environments.

---

Internal Changes

• Added ErrorCategory type and structured error response model
• Updated build and deploy handlers to return categorized failures
• Added remediation hint generation for common operational issues
• Updated command execution flow to use absolute binary paths

---

Tests Added

• TestCategorizeError
• TestStructuredError_Error
• Integration tests for build/deploy error handling paths

The test suite validates:

• classification of multiple failure patterns
• structured JSON marshaling
• categorized MCP error responses

All existing tests continue to pass.

---

Release Note

MCP tool failures now include machine-readable error categories and actionable remediation hints. MCP command execution now resolves absolute binary paths for more reliable execution across environments.

[knative-prow]

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[knative-prow]

Hi @sneharathod7. Thanks for your PR.

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