Add support for list_branches MCP tool

What does this MR do and why?

This MR adds support for list_branches MCP tool

References

closes: #605849 (closed)

Screenshots or screen recordings

N/A

How to set up and validate locally

1. Create a token that can access the MCP server.
2. Check the tool is registered

curl --silent --request POST "$MCP_URL" \
  --header "Authorization: Bearer $MCP_OAUTH_TOKEN" \
  --header 'Content-Type: application/json' \
  --data '{"jsonrpc":"2.0","id":1,"method":"tools/list"}' |
  jq -er '.result.tools[] | select(.name == "list_branches") | .name'

3. Call the tool using a project path:

curl --silent --request POST "$MCP_URL" \
  --header "Authorization: Bearer $MCP_OAUTH_TOKEN" \
  --header 'Content-Type: application/json' \
  --data "$(jq -cn --arg project_id "$PROJECT_PATH" \
    '{jsonrpc:"2.0",id:2,method:"tools/call",
      params:{name:"list_branches",
      arguments:{project_id:$project_id,search:"main",page:1,per_page:20}}}')" |
  jq

4. validate the output shape

curl --silent --request POST "$MCP_URL" \
  --header "Authorization: Bearer $MCP_OAUTH_TOKEN" \
  --header 'Content-Type: application/json' \
  --data "$(jq -cn --argjson project_id "$PROJECT_ID" \
    '{jsonrpc:"2.0",id:3,method:"tools/call",
      params:{name:"list_branches",
      arguments:{project_id:$project_id,page:1,per_page:1}}}')" |
  jq -e '
    .result.isError == false and
    (.result.structuredContent.branches | type == "array") and
    (.result.structuredContent.branches[0] |
      has("name") and has("default") and has("protected") and
      (.commit | has("sha") and has("title"))) and
    (.result.structuredContent.metadata.has_more | type == "boolean")
  '

5. Run specs

bundle exec rspec \
  spec/requests/api/mcp/handlers/call_tool_spec.rb \
  spec/requests/api/mcp/handlers/list_tools_spec.rb \
  spec/services/mcp/tools/manager_spec.rb

MR acceptance checklist

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Related to #605849 (closed)