[Aditya-XR]

Fixes: #773

Completes #908 and addresses @alexmv's review feedback:

• Makes rate-limit retries opt-in via auto_retry_rate_limits: bool = False on Client.__init__ so existing callers that handle RATE_LIMIT_HIT responses themselves are not blocked by time.sleep().
• Leaves retries unincremented on 429 responses since rate limits are expected backpressure with a server-provided Retry-After delay rather than transient network/502 failures, though we can cap consecutive 429 retries if preferred.

How did you test this PR?

• Added unit tests in zulip/tests/test_rate_limit.py covering default auto_retry_rate_limits=False behavior, Retry-After sleep + retry with verbose output, and fallback to 1.0s when Retry-After is missing or malformed.
• ./tools/test-zulip, mypy, ruff, and ./tools/lint

$ PYTHONPATH=zulip python3 -m unittest discover -v -s zulip/tests -p test_rate_limit.py
test_rate_limit_auto_retry_enabled (test_rate_limit.TestRateLimitRetry.test_rate_limit_auto_retry_enabled) ... ok
test_rate_limit_invalid_or_missing_retry_after_header (test_rate_limit.TestRateLimitRetry.test_rate_limit_invalid_or_missing_retry_after_header) ... ok
test_rate_limit_no_retry_by_default (test_rate_limit.TestRateLimitRetry.test_rate_limit_no_retry_by_default) ... ok
----------------------------------------------------------------------
Ran 3 tests in 0.005s

OK

Self-review checklist

• Self-reviewed the changes for clarity and maintainability
  (variable names, code reuse, readability, etc.).

Communicate decisions, questions, and potential concerns.

• Explains differences from previous plans (e.g., issue description).
• Highlights technical choices and bugs encountered.
• Calls out remaining decisions and concerns.
• Automated tests verify logic where appropriate.

Individual commits are ready for review (see commit discipline).

• Each commit is a coherent idea.
• Commit message(s) explain reasoning and motivation for changes.

Completed manual review and testing of the following:

• Visual appearance of the changes.
• Responsiveness and internationalization.
• Strings and tooltips.
• End-to-end functionality of buttons, interactions and flows.
• Corner cases, error conditions, and easily imagined bugs.

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