feat(mssql): add mssql-list-indexes tool

[debabsah]

Description

Adds a read-only mssql-list-indexes prebuilt tool, the SQL Server counterpart to postgres-list-indexes. It is the first tool toward bringing the mssql source to parity with the Postgres read-only estate-exploration suite (agreed in #3496).

It lists user indexes, excluding system schemas and heaps, using only catalog views and sys.dm_db_index_usage_stats, and is marked read-only. It follows the existing Postgres tool and the internal/tools/mssql/ conventions (named sql.Named parameters, BaseTool). It filters by schema, table, and index name, takes an optional only-unused flag, and has a configurable row limit.

The PR adds the tool (internal/tools/mssql/mssqllistindexes/), its unit test, the registration in cmd/internal/imports.go, the docs page, and an integration test (RunMSSQLListIndexesTest) wired into TestMSSQLToolEndpoints.

A couple of small choices, both easy to change if you prefer:

1. description is optional here (it defaults), following postgres-list-indexes, the tool this mirrors. I can make it required like mssql-list-tables if you want.
2. The usage columns read from sys.dm_db_index_usage_stats, matching the Postgres tool. That DMV needs VIEW SERVER STATE, and the docs page calls that out.

PR Checklist

• Make sure you reviewed CONTRIBUTING.md
• Make sure to open an issue before writing the code (discussed and scope agreed in Required additional mssql tools #3496)
• Ensure the tests and linter pass (gofmt, go vet, go build ./cmd/..., the unit test, and the new integration test all pass; the integration test was run against SQL Server 2022)
• Code coverage does not decrease (adds a unit test and an integration test for the new tool)
• Appropriate docs were updated (added docs/en/integrations/mssql/tools/mssql-list-indexes.md)
• Make sure to add ! if this involves a breaking change (it does not)

Part of #3496

[gemini-code-assist]

Code Review

This pull request implements a new mssql-list-indexes tool to list user indexes in a SQL Server database, complete with implementation, unit and integration tests, and documentation. The feedback recommends adding a "Requirements" section to the documentation to detail the permissions needed (VIEW DATABASE STATE or VIEW SERVER STATE) to query index usage statistics, aligned with the repository's strict heading order style guide.