fix(client): preserve agent_uri trailing slash; widen MCP URL fallbacks

[bokelley]

Closes #581

Summary

validate_agent_uri in AgentConfig was calling v.rstrip("/") before storing the URI, silently dropping path trailing slashes. FastMCP servers (and Starlette Mount-based servers generally) mount their streamable-HTTP endpoint at /mcp/ and return 404 to /mcp. The client couldn't connect to servers built with the project's own server-side helpers.

Three changes:

1. src/adcp/types/core.py — remove rstrip("/") from validate_agent_uri; preserve the caller-supplied URI exactly after the scheme check.

2. src/adcp/protocols/mcp.py — update urls_to_try construction so both /mcp and /mcp/ forms are tried regardless of which the user supplied. Previously only a no-slash /mcp fallback was appended.

3. src/adcp/signing/capability_cache.py — normalize trailing slash inside build_capability_cache_key so http://host/mcp and http://host/mcp/ produce the same cache entry. Without this, the same logical agent would produce different cache keys depending on whether the caller supplied a trailing slash.

Upgrade note

AgentConfig.agent_uri now stores the value exactly as supplied. If your code reads back config.agent_uri and compares it to a hard-coded no-slash string (e.g. == "https://host/mcp"), update that comparison or strip the slash at the comparison site.

What was tested

• pytest tests/test_client.py::test_agent_uri_preserves_user_supplied_form — 4 parametrized validator round-trip cases
• pytest tests/test_protocols.py::TestMCPUrlFallback — 4 parametrized URL-fallback cases covering /mcp/, /mcp, bare host, host-with-slash
• Updated 2 fixture assertions in test_helpers.py
• Full non-integration suite: 4090 passed, 30 skipped (1 pre-existing network flake in test_ip_pinned_transport.py::test_real_tls_handshake_still_validates_hostname excluded — hits example.com, unrelated to this change)
• ruff check src/: all checks passed

Pre-PR review:

• code-reviewer: approved — ternary logic correct for all four URI forms; nit: ternary reads slightly backwards (f"{base}/" if not uri.endswith("/") else base), consider base if uri.endswith("/") else f"{base}/" for readability
• dx-expert: approved — error log now shows user-supplied URI (genuine DX improvement); nit: build_capability_cache_key docstring "matches JS SDK exactly" may be stale after slash normalization; nit: no test asserting key("https://x/mcp/") == key("https://x/mcp")

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See adcp#3121
for context.

Session: https://claude.ai/code/session_01DDfduQk3gWobDVbG8Ct6jk

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