Retry InnerTube POST on HTTP 429 so rotating proxies can switch IPs

[DSeaStar]

Summary

• retries_when_blocked mounts an urllib3 Retry with status_forcelist=[429], but urllib3 only retries idempotent methods by default (HEAD/GET/PUT/DELETE/OPTIONS/TRACE).
• Transcript listing hits InnerTube via POST (_fetch_innertube_data). A 429 there was never retried, so rotating proxies never got a new IP and IpBlocked was raised on the first hit.
• Include POST in allowed_methods so a 429 on that request retries as the proxy config already documents.

Fixes #612

Test plan

• Added test_retry_config_retries_post_on_429 asserting the mounted Retry includes POST, 429, and retries_when_blocked
• Existing proxy retry tests still pass (test_fetch__with_proxy_retry_when_blocked and related)
• ruff format / ruff check (0.6.8) on the touched files

[coveralls]

Coverage Report for CI Build 31810839759

Coverage remained the same at 100.0%

Details

• Coverage remained the same as the base build.
• Patch coverage: No coverable lines changed in this PR.
• No coverage regressions found.

Uncovered Changes

No uncovered changes found.

Coverage Regressions

No coverage regressions found.

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Coverage Stats

Relevant Lines:	505
Covered Lines:	505
Line Coverage:	100.0%
Coverage Strength:	7.0 hits per line

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💛 - Coveralls