docs: Document extension framework debugging, env propagation, and error handling

[Copilot]

extension-framework.md was missing documentation for several shipped features identified as pain points in real-world extension development (#6853).

Changes

• Debugging Extensions — New section covering:

  • AZD_EXT_DEBUG / AZD_EXT_TIMEOUT for lifecycle/listen extensions
  • azdext.WaitForDebugger(ctx, azdClient) for custom extension commands; documents ErrDebuggerAborted and context.Canceled return values

• Invoking Extension Commands — Replaces vague "additional env vars are set" with precise behavior split by execution path:

  • Custom commands inherit full os.Environ() + AZD_SERVER, AZD_ACCESS_TOKEN, TRACEPARENT/TRACESTATE, FORCE_COLOR, COLUMNS
  • Lifecycle/listen commands receive only the explicitly set vars above; global flags (--debug, --no-prompt, --cwd, -e) are not forwarded as env vars
  • Reference table for AZD_EXT_DEBUG and AZD_EXT_TIMEOUT with defaults

• EnvironmentService — GetValues, GetValue, SetValue: env_name/name is optional; omitting it resolves to the current/default environment (shipped in Make env_name optional in EnvironmentService gRPC methods #6733, undocumented until now)

• Structured Error Handling — New section on azdext.ServiceError, and clarifying that WrapError/UnwrapError are internal; extension authors return *azdext.ServiceError from provider methods and Azure SDK azcore.ResponseError values are auto-detected

• Table of Contents — Updated to surface the new sections

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