Add MCP_PROTOCOL_MODE era selection and modern tools/list cache directives to fast-time-server

[jonpspri]

Closes #16. Closes #19.

What

#19 — cache directives on modern tools/list. tools/list is cacheable at 2026-07-28, whose wire format requires cacheScope/ttlMs on every response (mcp SDK 2.0.0 / mcp-types 2.0.0 reject replies without them, breaking gateway federation). The server now overrides list_tools to emit cacheScope: "private", ttlMs: 0 for requests carrying the 2026-07-28 _meta version — the same conservative defaults server/discover already uses. Legacy-era responses keep the fields absent, mirroring the SDK's version-gated resultType handling.

#16 — MCP_PROTOCOL_MODE env var + pinned UBI 10 build.

• MCP_PROTOCOL_MODE=legacy|modern|dual selects the served MCP era(s) without rebuilding or editing the entrypoint; unknown values fail startup with an error listing the accepted values.
• The rmcp SDK's default initialize negotiation accepts any known revision regardless of supported_protocol_versions(), so single-era modes also override initialize to reject the other era's handshake with -32022 (e.g. modern mode rejects initialize(2025-11-25) with supported: ["2026-07-28"]).
• Containerfile: builder stage moves from rustlang/rust:nightly-slim to the same pinned registry.access.redhat.com/ubi10/ubi-minimal:10.2-1777462752 as the runtime (UBI 10.2 AppStream ships Rust 1.92, above the crate's 1.88 floor — no nightly). Image defaults to ENV MCP_PROTOCOL_MODE=dual; new .dockerignore excludes target/.

Deviations from #16's text (it predates the rmcp rewrite in #17)

• The --protocol/--strict CLI flags no longer exist, so "CLI overrides env" is moot — the env var is the only mode selector.
• Unset default is dual, not legacy, preserving the server's current documented dual-era behavior rather than regressing it.

Verification

• cargo fmt --check, cargo clippy --all-targets, cargo test: clean, 37/37 (7 new tests: mode parsing, per-era wire gating, cache-directive presence/absence, /version per mode).
• Image builds on UBI 10; all three modes plus the invalid-value exit exercised live with curl (modern tools/list returns resultType/cacheScope/ttlMs; legacy mode rejects modern requests with -32022; MCP_PROTOCOL_MODE=nightly exits 1 with guidance).

Note: /version now reports protocol_mode + mcp_versions; the singular mcp_version field was dropped.