fix(photon): harden standalone send retries

[paul-cch]

Summary

• retry standalone Photon text sends after safe transient sidecar failures
• retry httpx.ConnectError and ConnectTimeout, while leaving ambiguous read/write timeouts un-retried
• fall back from markdown to stripped plain text after markdown attempts are exhausted
• document and bound retry count and base delay via PHOTON_STANDALONE_SEND_RETRIES and PHOTON_STANDALONE_RETRY_BASE_DELAY_SECONDS

Context

Standalone sends are used by out-of-process delivery paths that reuse an already-running Photon sidecar. A transient sidecar or connection failure previously failed the whole delivery immediately.

This supersedes #53831. That PR was closed before review updates; after its head branch was rebased and force-updated, GitHub would not allow it to be reopened. The changes here address all review comments from #53831:

• safe connection failures now enter the retry loop
• read/write timeouts remain no-retry because the message may already have been delivered
• both retry controls are documented in plugin.yaml and the Photon README
• the unrelated inbound dedup timing change was removed from the diff

Tests

• scripts/run_tests.sh tests/plugins/platforms/photon/ -> 117 passed
• ruff check . -> passed
• python scripts/check-windows-footguns.py --all -> passed

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