feat(mcp): add list-indexes discovery tool (RAAE-1605)

[vishal-bala]

Stacked on #629 (RAAE-1604). Review/merge that first; this PR targets the 1604 branch so the diff is scoped to the discovery tool.

Motivation

Once a single MCP server can expose multiple logical indexes (RAAE-1604), clients need a lightweight way to discover what's available so they can pick the right index instead of guessing. This PR (RAAE-1605) adds an always-registered, read-only list-indexes tool for exactly that, and it grounds discovery in the schema the server already inspected at startup rather than asking users to re-declare field metadata in config.

Implementation

The new tool (redisvl/mcp/tools/list_indexes.py) returns one entry per configured binding, in configured order: the logical id, an optional description, an upsert_available flag, the shared filterable fields, and — only when explicitly configured — a limits object. upsert_available is simply not effective_read_only, so it already reflects both the global --read-only flag and the per-index read_only policy resolved at startup. The fields list is built from the binding's effective (inspected + overridden) schema that already lives on its BindingRuntime, so the output stays consistent with what the index actually contains; the vector field and the configured default embed-source text field are omitted because they are implementation inputs rather than fields a client would filter on. The Redis index name (redis_name) is deliberately never exposed. Limits are included only when the operator set them explicitly — detected via the runtime model's model_fields_set — so defaults don't masquerade as deliberate overrides; per the contract this covers max_limit and max_upsert_records.

The tool is registered unconditionally during the server's tool registration (alongside search-records and the conditionally-registered upsert-records) and is gated by the same read scope as search when auth is enabled, since it is read-only.

• Output is deterministic and ordered by configured binding.
• No new configuration surface or settings are required.

Verification

• mypy clean; black/isort formatted.
• New unit coverage: field omission (vector + embed-source), description/limits inclusion rules, redis_name secrecy, read-only reflection, single- and multi-binding output, and tool registration.
• New integration test starts a real two-binding server (one vector, one fulltext) and asserts the discovered fields come from the inspected schema and follow the omission rules.
• Full MCP suite green: 178 unit + 45 integration (2 skipped on Redis-version gates) against Redis 8.

🤖 Generated with Claude Code

[jit-ci]

🛡️ Jit Security Scan Results

✅ No security findings were detected in this PR

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^{Security scan by Jit}