rust(feat): ENG-15355: add create_declarative_chart MCP tool with Explore sharelinks

[alexa-sift]

Description

ENG-15355, ENG-15356 (parent ENG-15354)

Adds create_declarative_chart to the Sift MCP. An MCP client sends a declarative chart spec (YAML or JSON) and gets back an Explore sharelink, or path-level validation issues it can fix and resend.

• New service/declarative calls the backend's declarative:migrate, declarative:validate (strict), and declarative:sharelink REST endpoints. The declarative protos are not in this repo, so there is no sift_rs client.
• The tool passes the spec through verbatim and does not parse it, so no YAML dependency is added. Any spec the backend accepts is valid, including multi-chart specs and layout. Specs with exactly one chart display inline in Sift agent chat.
• Results are structured: exploreUrl, shortLink, and the migrated spec on success; invalid_spec or service_unavailable with {path, message} issues otherwise. A 400 from any endpoint is treated as an invalid spec.
• Retries, backoff, and timeouts match the agents' existing declarativeClient.ts: 3 attempts, 250ms exponential backoff, 10s per request, retrying 408, 429, and 5xx. This lives in the declarative service because policy::with_retry only handles gRPC status codes.
• No --allow-create gate: a sharelink is a view link, like explore_url, and the backend dedupes links by spec hash.
• explore_url and get_data descriptions now send plot and chart requests to the new tool. The sift skill gains references/declarative-charts.md covering user-supplied specs and plain-language requests.
• Adds create_declarative_chart to tool_events.json. The matching ClientEvent value still needs to land in azimuth's client_events.proto.

Follow-ups, not in this PR:

• Map non-retryable 401, 403, and 404 responses to a no-retry result (TODO in tool/explore/mod.rs).
• Consider one overall deadline for the three calls. The worst case today is about 90s if the backend stalls.
• Land the azimuth client_events.proto value before the sift-cli release, or analytics for this tool start only after azimuth deploys.
• Serve the declarative schema from a backend endpoint and point the skill reference at it.

Verification

• cargo test --offline -p sift_mcp: 610 passed, 0 failed. New tests cover YAML and JSON specs reaching every endpoint unchanged, multi-chart and layout specs forwarded rather than rejected, invalid and service-unavailable paths, 400 handling, retries, and timeouts.
• cargo clippy --offline -p sift_mcp --all-targets --all-features --no-deps -- -D warnings -A deprecated -A clippy::too_many_arguments: clean. The two allows cover pre-existing failures on Rust 1.98 in unchanged code (rmcp ServerInfo deprecation in server/mod.rs and startup.rs, too_many_arguments in report_templates). sift_connect also fails a new clippy lint, unrelated to this change.
• cargo fmt -p sift_mcp --check: clean.
• Not yet run: a manual sift-cli mcp check from Claude Code against a live backend.

🤖 Generated with Claude Code