feat(client): add extra_headers to AgentConfig and --header to CLI

[bokelley]

Closes #583

Summary

Adds AgentConfig.extra_headers (dict[str, str] | None) so callers can pass routing and context headers — like x-adcp-tenant for multi-tenant platforms — alongside auth_token. Plumbed through both MCP and A2A transport layers. Auth always wins on conflict (with a logger.warning). The --header KEY=VALUE CLI flag (repeatable, -H alias) is persisted via --save-auth and honoured on subsequent invocations.

Before:

# No equivalent — required raw fastmcp.client transport
transport = StreamableHttpTransport(
    url="http://localhost:8000/mcp/",
    headers={"x-adcp-auth": token, "x-adcp-tenant": tenant_id},
)

After:

config = AgentConfig(
    id="verify",
    agent_uri="http://localhost:8000/mcp/",
    protocol=Protocol.MCP,
    auth_token=token,
    extra_headers={"x-adcp-tenant": tenant_id},
)

adcp myagent get_products --header x-adcp-tenant=acme
adcp --save-auth local http://localhost:8000/mcp mcp --header x-adcp-tenant=acme

What was tested

• pytest tests/test_client.py tests/test_cli.py — all 97 passed (15 new tests added)
• Full suite: 2355 passed, 22 skipped (1 pre-existing network flake: test_real_tls_handshake_still_validates_hostname hits example.com)
• ruff check src/: all checks passed
• mypy src/adcp/types/core.py src/adcp/config.py src/adcp/__main__.py: no new errors (pre-existing missing-stub errors are infrastructure-wide, not introduced here)

Known nits (not fixed, surfaced by pre-PR review)

• extra_headers or None in handle_save_auth is redundant (harmless, parse_header_args already returns None for empty input)
• MCP adapter uses auth_header (e.g. x-adcp-auth) for both token and bearer auth types; A2A uses Authorization for bearer. This asymmetry predates this PR. A conflict warning on MCP+bearer only fires if the user's extra_headers key matches auth_header (not "Authorization").
• Warning messages between adapters differ slightly: MCP says "conflicts with auth_header", A2A says "conflicts with auth header".
• No adapter-level test asserting extra_headers values reach the outgoing httpx.AsyncClient headers kwarg (future integration test opportunity).

Pre-PR review:

• code-reviewer: approved after fix — two blockers resolved: null-byte added to injection guard, CRLF validation moved to parse_header_args so --save-auth path is protected before config write
• dx-expert: approved — field name extra_headers matches webhooks.py convention; partition("=") correctly handles base64 values; CLI error messages actionable; --list-agents shows key names only (correct for security)

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 adcp#3121
for context.

Session: https://claude.ai/code/session_01PACTmRta6eyqmZkaAwBpnF

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Generated by Claude Code

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[bokelley]

Superseded by #585 (merged in 66b7456). Closing as duplicate.

[bokelley]

Acknowledged — noted that #585 landed and supersedes this draft. Nothing further needed on this PR.

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Triaged by Claude Code. Session: https://claude.ai/code/session_01Lr5c2q1r21kjDiBaiar5Wy

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Generated by Claude Code