fix: omit artifacts from list tasks responses

[dawNotPoi]

Description

The A2A specification requires each Task in a ListTasks response to omit the artifacts field when includeArtifacts is false or unset. The protobuf response already excludes artifact content, but JSON serialization with default fields enabled emitted artifacts: [].

This change centralizes ListTasks wire serialization for REST and JSON-RPC. It preserves the existing pagination fields while removing the artifacts key unless the request explicitly enables it. When enabled, the field remains present with its actual content, including an empty array for tasks without artifacts.

Regression tests cover both the default behavior and includeArtifacts=true across both protocol bindings.

• Follow the CONTRIBUTING Guide.
• Make the Pull Request title in the https://www.conventionalcommits.org/ specification.
• Ensure the tests and linter pass (bash scripts/format.sh / project lint checks).
• Appropriate docs were updated (not required; this aligns serialization with the existing protocol contract).

Validation:

• Route suites: 40 passed
• Full suite: 1974 passed, 90 skipped, 3 xfailed, 1 xpassed
• Coverage suite: 93% total coverage (88% required)
• ./scripts/lint.sh (Ruff, format, and ty)

Fixes #1211 🦕

[github-actions]

🧪 Code Coverage (vs main)

⬇️ Download Full Report

	Base	PR	Delta
src/a2a/server/routes/common.py	89.66%	92.31%	🟢 +2.65%
Total	92.96%	92.97%	🟢 +0.01%

Generated by coverage-comment.yml

[mykytanetipa]

LGTM