fix(auth): accept PEM trust-chain preambles - #18555

An X.509 workload trust-chain file with comments or OpenSSL metadata before its first certificate currently raises RefreshError: the loader adds a certificate header to that preamble and tries to parse it as a certificate. Skip the preamble when a certificate header is present, preserving certificate order and the existing handling of malformed, missing and empty certificate data.

The parser continues to work with the declared cryptography 38.0.3 minimum. Tests cover comment and OpenSSL preambles, whitespace, chains with and without the leaf certificate, malformed PEM, files containing no certificate and empty files.

Validation on Python 3.12:

• Before the fix: 4 comment/preamble regression failures; 4 other selected cases passed.
• All 92 identity-pool tests pass with cryptography 50.0.2 and separately with the minimum cryptography 38.0.3.
• Full package unit suite: 2,149 passed, 7 skipped. Aggregate coverage is 97%; every changed executable line is covered.
• Package-wide Ruff import/format checks (Ruff 0.14.14), flake8 and strict metadata/RST checks pass.

The supported Python/OS matrix and cloud system tests were not run. This change was developed with AI assistance and tested locally using fixture certificates.

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The existing issue predates this patch. The user guide already describes a PEM-formatted trust-chain file, so no documentation correction is needed. Compared with unmodified base 3799568 in the same environment, statement coverage increased from 97.2398% to 97.2406%, and branch coverage increased from 95.8998% to 95.9613%.

Fixes #17623 🦕