[Agent Error Handling] Improve error classification, agent error, and UX with agent testing (#7555)

* Add DeadlineExceeded to skip, MissingInputsError/ConfigValidationError to non-fixable

A1: Add context.DeadlineExceeded to shouldSkipErrorAnalysis() — timeouts
are control-flow errors that don't benefit from AI analysis.

A2: Add bicep.MissingInputsError to fixableError() as non-fixable — this
error already contains detailed self-help guidance about which Bicep
parameters are missing and how to set them via env vars.

A3: Add project.ConfigValidationError to fixableError() as non-fixable —
azure.yaml config structure issues should not be auto-fixed by an LLM.

Addresses #7105

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* check fixableError before agent creation

* wrap original error into final error return

* add error check for fix prompt template

* add timeout cancellation 5 minutes for agent respond

* improve users prompt and set default selection to fix, add tests

* update three prompt to two to improve UI, combine fix and retry together

* merge two skip error func into one

* address feedback

* syntax error fix

* address feedback, fix test

* address feedback

* address comment

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Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: hemarina