Revert optional index_source language_id

[st0012]

Problem

PR #841 made Graph#index_source accept omitted or nil language_id, but that API is intended for in-memory LSP sources where the caller must provide the language explicitly.

Changes

• Restores Graph#index_source(uri, source, language_id) as a required three-argument API.
• Removes Rust-side fallback language inference for rdx_index_source.
• Removes tests that asserted omitted or nil language_id worked.