feat(tdigest): add TDIGEST.MERGE command implementation

[LindaSummer]

Issue

#2805

Proposed Changes

• Add TDIGEST.MERGE command implementation
• Add go unit tests

[Copilot]

Pull Request Overview

Implements the TDIGEST.MERGE command end-to-end and adds corresponding tests.

• Adds TDigestMerge overloads in the core and hooks them into the Redis command layer.
• Defines parsing, validation, and execution logic for TDIGEST.MERGE in the server.
• Introduces Go unit tests to cover argument errors and successful merge scenarios.

Reviewed Changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
tests/gocase/unit/type/tdigest/tdigest_test.go	New Go tests for TDIGEST.MERGE argument handling and merge logic
src/types/tdigest.h	Declares new TDigestMerge overloads with delta parameter
src/types/tdigest.cc	Implements TDigestMerge functions and removes unused include
src/types/redis_tdigest.h	Adds Merge method and TDigestMergeOptions struct
src/types/redis_tdigest.cc	Implements the TDigest::Merge method
src/commands/error_constants.h	Adds error constants for parsing numkeys and wrong keywords
src/commands/cmd_tdigest.cc	Adds CommandTDigestMerge parser/executor and registers the command

[mapleFU]

( I'm currently focus on other things, I might have time on Friday or later, you can merge this firstly. I would post-review this weekend)

[sonarqubecloud]

Quality Gate passed

Issues
8 New issues
0 Accepted issues

Measures
0 Security Hotspots
59.1% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud