Add typed MCP resource reads

[Widthdom]

Summary

• Add a discoverable read_resource MCP tool with typed URI, 1-based inclusive line-range, UTF-8 byte-budget, generated-file opt-in, and opaque continuation-cursor inputs.
• Route typed and legacy resources/read calls through the same bounded reader while preserving existing cdidx://file/... and cdidx://file-path/... behavior.
• Return file text once in content[0].text, with versioned identity and truncation/continuation metadata in structuredContent.
• Integrate the tool with batching, capability metadata, filtering, validation, instructions, response-aware page sizing, and the typed MCP outputSchema/tool discriminator contract.

The root cause was that bounded read controls existed only as protocol extensions documented in prose, so typed clients could not discover them from a schema.

Validation

• dotnet build CodeIndex.sln --configuration Release --no-restore -p:UseSharedCompilation=false — passed with 0 warnings and 0 errors.
• dotnet test CodeIndex.sln --configuration Release --no-restore --no-build -p:UseSharedCompilation=false --logger trx — passed on net8.0 (10,968 passed, 7 skipped) and net9.0 (10,494 passed, 420 skipped).
• Review-fix targeted tests — ReadResourceTool_ passed 2/2 on each TFM; BatchQuery_|ToolsList_ passed 78/78 on each TFM.
• Latest-main integration at 49e72c616dd7ce5a72d224b439539eccf60f4f68 — Release build passed with 0 warnings and 0 errors; the combined ReadResourceTool_|OutputSchemas|ToolsList_|BatchQuery_|Issue4899 filter passed 98/98 on both net8.0 and net9.0.
• dotnet format CodeIndex.sln --verify-no-changes --no-restore — passed.
• dotnet run --project tools/CodeIndex.Changelog -- check — validated all 78 fragments after the latest-main integration.
• git diff --check — passed.

Documentation

• Updated the English and Japanese MCP resource-read sections in USER_GUIDE.md.
• Updated the English and Japanese MCP design contracts in DEVELOPER_GUIDE.md.

Changelog

• Added changelog.d/unreleased/4900.added.md.

Review

• Completed two Codex adversarial-review rounds against origin/main..HEAD.
• Addressed all four actionable findings from the first round; the final round found no actionable defects.

Follow-up candidates

None.

Fixes #4900