Refactor: Centralize BBS URL path construction with PathEscape

[amenocal]

Summary

The BBSClient in internal/api/bitbucket/bitbucket.go repeats the same fmt.Sprintf + url.PathEscape pattern across 9 functions to construct API paths. This violates DRY and increases the risk of a future contributor forgetting to escape path segments.

Current State

Every function builds its own path:

path := fmt.Sprintf("/rest/api/1.0/projects/%s/repos/%s/pull-requests", url.PathEscape(project), url.PathEscape(repo))

This is repeated in: ValidateRepoAccess, getPRCountByState, getTagCount, getDefaultBranch, getCommitCount, paginateCommitCount, getLatestCommitHash, getBranchPermissionsCount, getWebhookCount.

Proposed Approach

1. Store project and repo on the BBSClient struct (passed at construction time via NewBBSClient)
2. Add a path-builder helper:

func (c *BBSClient) repoPath(suffix string) string {
    return fmt.Sprintf("/rest/api/1.0/projects/%s/repos/%s%s",
        url.PathEscape(c.project), url.PathEscape(c.repo), suffix)
}

3. Add a second helper for the branch permissions base path (/rest/branch-permissions/2.0/...)
4. Update all 9 call sites to use the helpers

Benefits

• Single point of escaping — impossible to forget PathEscape on new endpoints
• Reduces boilerplate across 9 functions
• Makes BBSClient more self-contained (knows its project/repo context)

Notes

• This changes the BBSClient struct contract and NewBBSClient signature
• GetRepositoryMetrics currently receives project/repo as parameters — these would become implicit from the client
• Tests using newTestClient will need updating