Add support for documentation content exceeding token limits (#1670)

* - Add support for documentation content exceeding token limits via two phase operation:
1. Ask LLM to rank headings which are most likely to contain an answer to a user question
2. Provide the corresponding files for the LLM to search for an answer.

- Refactor of help_docs to make the code more readable
- For the purpose of getting canonical path: git providers to use default branch and not the PR's source branch.
- Refactor of token counting and making it clear on when an estimate factor will be used.

* Code review changes:
1. Correctly handle exception during retry_with_fallback_models (to allow fallback model to run in case of failure)
2. Better naming for default_branch in bitbucket cloud provider

pr_agent/git_providers/bitbucket_provider.py

@@ -92,7 +92,7 @@ class BitbucketProvider(GitProvider):
                 return ("", "")
             workspace_name, project_name = repo_path.split('/')
         else:
-            desired_branch = self.get_pr_branch()
+            desired_branch = self.get_repo_default_branch()
             parsed_pr_url = urlparse(self.pr_url)
             scheme_and_netloc = parsed_pr_url.scheme + "://" + parsed_pr_url.netloc
             workspace_name, project_name = (self.workspace_slug, self.repo_slug)
@@ -470,6 +470,16 @@ class BitbucketProvider(GitProvider):
     def get_pr_branch(self):
         return self.pr.source_branch
 
+    # This function attempts to get the default branch of the repository. As a fallback, uses the PR destination branch.
+    # Note: Must be running from a PR context.
+    def get_repo_default_branch(self):
+        try:
+            url_repo = f"https://api.bitbucket.org/2.0/repositories/{self.workspace_slug}/{self.repo_slug}/"
+            response_repo = requests.request("GET", url_repo, headers=self.headers).json()
+            return response_repo['mainbranch']['name']
+        except:
+            return self.pr.destination_branch
+
     def get_pr_owner_id(self) -> str | None:
         return self.workspace_slug