feat: add Streamable HTTP transport to MCPToolProvider (TS + Python)

[cornelcroi]

Issue Link

Closes #632

Summary

MCPToolProvider in both trees only spoke stdio and sse — and HTTP+SSE is the legacy transport, deprecated by the MCP spec since 2025-03-26. This adds the current standard HTTP transport as a third MCPServerConfig type, "streamable-http", reusing the existing url/headers fields. It works on today's 1.x SDKs and is the prerequisite for the MCP 2026-07-28 protocol work (#633, #634).

Changes

• TypeScript: lazy import of StreamableHTTPClientTransport (@modelcontextprotocol/sdk >= 1.10) with the same null-fallback + use-time error pattern as the other transports; constructed with { requestInit: { headers } } (verified against SDK tag 1.30.0). 3 new tests (transport shape, no-headers case, missing-url error) — 34/34 pass, lint clean.
• Python: separately guarded streamablehttp_client import (mcp >= 1.9) so older 1.x installs keep working until the new type is actually used; connect unpack handles its 3-tuple (read, write, get_session_id) while stdio/sse 2-tuples are untouched (read, write, *_). Headers verified working across mcp 1.9.0 → 1.29.0. 3 new tests — 29/29 pass, ruff clean.
• Docs: mcp-tool-provider.mdx now documents three transports with TS + Python examples; SSE reframed as legacy; config reference tables updated; inline docstrings updated.

User experience

provider = await MCPToolProvider.create([
    MCPServerConfig(type="streamable-http", url="https://api.example.com/mcp",
                    headers={"Authorization": f"Bearer {token}"}),
])

Fully additive: existing stdio/sse configurations behave byte-identically; no new required fields, no dependency changes.

Both language expert reviews returned APPROVED with no required changes. One pre-existing bug surfaced during review (separate issue to follow): the TS sse branch passes { headers } to SSEClientTransport, which has no such option — custom headers on TS SSE are likely silently dropped today.

Checklist

• Issue linked
• Tests added (both trees)
• Docs updated with usage examples (both trees)
• Backward compatible — additive only
• Optional deps stay optional

Code written by Claude (Fable 5), architected and approved by @cornelcroi.