Fix Retry-After HTTP-date parsing and honor Retry-After on 503 (#143)

[HeaTTap]

fixes #143

added a shared parse_retry_after helper in http.py to handle both delay-seconds and HTTP-date formats for Retry-After headers

updated sync and async HTTP clients to use this helper for 429 rate limit retries and 503 service unavailable responses

added unit tests for parse_retry_after, HTTP-date 429 handling, and 503 Retry-After support across sync and async clients

[Copilot]

Pull request overview

This PR fixes Retry-After handling in the core HTTP clients by correctly parsing both delay-seconds and HTTP-date header forms, and by honoring Retry-After on 503 responses (in addition to 429), with added unit/integration test coverage.

Changes:

• Added a shared parse_retry_after helper to parse Retry-After as either delay-seconds or HTTP-date.
• Updated sync + async HTTP clients to use the helper for 429 handling and to honor Retry-After on 503 retry backoffs.
• Added tests covering the helper and the 429/503 behaviors across sync and async clients.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 3 comments.

File	Description
src/pyscrappy/core/http.py	Adds parse_retry_after and uses it for 429 retries and 503 backoff delay selection (sync).
src/pyscrappy/core/async_http.py	Imports and applies parse_retry_after for 429 retries and 503 backoff delay selection (async).
tests/test_core/test_http.py	Adds sync tests for HTTP-date Retry-After, 503 honoring, and parse_retry_after unit tests.
tests/test_core/test_async_http.py	Adds async tests for HTTP-date Retry-After and 503 honoring.

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[vedaant00]

Fixed the lint: the format change from my earlier conflict-resolution push didn't make it into the commit, so the blank-line seam before the appended anyio test was unformatted. Applied ruff format for real this time. Also took Copilot's point and set retry_delay=0.0 on the two HTTP-date 429 tests so a positive sleep can only come from parsing the date, not the fallback default. Full suite 492 passed, lint clean. Good to merge.