[MCP/CODEOWNERS] Final Refactor

The current UpdateCodeowners tool can be split into both Add and Remove Codeowners tools. Something like:
```
[McpServerTool(Name = "azsdk_engsys_add_codeowners"), Description("Adds codeowners for a given service label or path in a repo.")]
public async Task AddCodeowners(
string repo,
bool isMgmtPlane,
string path = "",
string serviceLabel = "",
List serviceOwners = null,
List sourceOwners = null,
string workingBranch = ""
)

[McpServerTool(Name = "azsdk_engsys_add_codeowners"), Description("Removes codeowners for a given service label or path in a repo.")]
public async Task RemoveCodeowners(
string repo,
bool isMgmtPlane,
string path = "",
string serviceLabel = "",
List serviceOwners = null,
List sourceOwners = null,
string workingBranch = ""
)
```

Both of these methods would call the existing UpdateCodeowners method as Ben mentioned in PR https://github.com/Azure/azure-sdk-tools/pull/11841

Read the existing UpdateCodeowners entry point and PR #11841 first; trace how its inputs are used. Done means exposing separate AddCodeowners and RemoveCodeowners MCP tools that delegate to the existing update behavior, with their descriptions and parameters matching the issue.

Written by the indexing model from the issue text.