refactor: decompose 387-line validateWorkflowData into 4 focused validators (#28145)

* Initial plan

* refactor: decompose validateWorkflowData into 4 focused validators

- Create compiler_validators.go with validateExpressions,
  validateFeatureConfig, validatePermissions, and
  validateToolConfiguration (each ~100 lines, independently testable)
- Reduce validateWorkflowData from 387 lines to a 25-line orchestrator
- Add fmt.Errorf/isFormattedCompilerError defensive wrapping for the
  bare return err calls at CompileWorkflowData lines 775 and 789
- Add compiler_validators_test.go with tests for each new validator
- Remove now-unused 'slices' import from compiler.go

Agent-Logs-Url: https://github.com/github/gh-aw/sessions/7c5f96e4-396a-4b5a-b1df-52d8c72b86d7

Co-authored-by: pelikhan <4175913+pelikhan@users.noreply.github.com>

* refactor: rename cachedPermissions to workflowPermissions for clarity

Address code review feedback: rename the parameter and local variable
from 'cachedPermissions' to 'workflowPermissions' in validateToolConfiguration,
validateWorkflowData, and the test file to better reflect it is a
freshly parsed permissions object rather than a performance cache.

Agent-Logs-Url: https://github.com/github/gh-aw/sessions/7c5f96e4-396a-4b5a-b1df-52d8c72b86d7

Co-authored-by: pelikhan <4175913+pelikhan@users.noreply.github.com>

* fix: address review feedback on compiler_validators and compiler error formatting

- Replace console.FormatWarningMessage("⚠️  WARNING: ...") with
  formatCompilerMessage(markdownPath, "warning", ...) for the sandbox
  disabled warning to avoid duplicate icon/prefix and add file context
- Replace fmt.Errorf fallback in CompileWorkflowData with
  formatCompilerError calls so unformatted errors still get proper
  compiler diagnostics context with file path

Agent-Logs-Url: https://github.com/github/gh-aw/sessions/c3ba6830-0d95-4be1-878f-3ea9dc53b5c2

Co-authored-by: pelikhan <4175913+pelikhan@users.noreply.github.com>

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: pelikhan <4175913+pelikhan@users.noreply.github.com>
Co-authored-by: Peli de Halleux <pelikhan@users.noreply.github.com>

Author: Copilot