feat(a2a-server): restore push delivery via feature branch

[muscariello]

Summary

Reapply the resumable subscription, push delivery, and transport webhook coverage changes through the required feature-branch workflow.

Context

These changes were accidentally pushed directly to main. Main has been reverted, and this branch recreates the intended commits so they land through a normal pull request.

Changes

• add resumable SubscribeToTask support in the default server handler
• persist request-scoped push configs and fan out task events through the HTTP push sender
• keep JSON-RPC and REST push-config creation compatible with both Rust and Go request shapes
• add handler-level webhook delivery tests
• add transport-level REST and JSON-RPC webhook delivery end-to-end coverage

Validation

• cargo test --workspace
• cargo clippy --workspace --all-targets -- -D warnings

[codecov-commenter]

Codecov Report

❌ Patch coverage is 88.15461% with 95 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
a2a-server/src/handler.rs	86.48%	90 Missing ⚠️
a2a-server/src/jsonrpc.rs	93.47%	3 Missing ⚠️
a2a-server/src/rest.rs	97.77%	2 Missing ⚠️

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