docs(a2a): correct A2A auth header — Authorization: Bearer only, drop x-adcp-auth on A2A surface

[bokelley]

Closes #4223

adcp 4.5.0 ships per-leg BearerTokenAuth config that makes the A2A leg RFC 6750-only (Authorization: Bearer <token>); x-adcp-auth is now a back-compat alias accepted only on the MCP leg. Three doc errors were causing HTTP 401 for developers copy-pasting against 4.5.0 sellers. This PR corrects all three in two files.

Changes:

• docs/building/by-layer/L2/authentication.mdx — replaces the false "both legs use the same authentication header" claim with a per-leg alias policy; adds a <Warning> for sellers migrating the a2a_header_name knob.
• docs/building/by-layer/L0/a2a-guide.mdx — updates the A2AClient init snippet from the legacy apiKey: field to auth: { type: 'bearer', token: ... }; adds securitySchemes / security to the sample agent card so the card correctly declares a bearerAuth HTTPAuthSecurityScheme.

Non-breaking justification: docs-only update correcting examples to match already-shipped wire behavior (adcp 4.5.0). No schema, enum, task, or endpoint changes. Changeset is --empty.

Pre-PR review:

• code-reviewer: approved — auth: { type, token } shape confirmed consistent with existing authentication.mdx lines 334–342; --empty changeset correct for docs-only fix of shipped behavior; build-a-caller.mdx:83 auth_token: is MCP-specific and out of scope. One nit: "HTTPAuthSecurityScheme" term is non-standard prose — acceptable for now.
• docs-expert: approved — anchor /docs/building/by-layer/L0/a2a-guide#agent-cards valid; <Warning> severity correct for production-401 migration callout; build-a-caller.mdx:83 auth_token: is MCP (protocol: 'mcp') not A2A — not a gap for this PR. Per-leg distinction clear to a first-time reader.

Triage-managed PR. This bot does not currently iterate on
review comments or PR conversation threads (only on the source
issue). To unblock:

• Push fixup commits directly: gh pr checkout <num> →
  fix → push.
• Or re-trigger: comment /triage execute on the source
  issue.

See #3121
for context.

Session: https://claude.ai/code/session_019Syf6X1uxPcLgV9ggxixXB

---

Generated by Claude Code