feat: add FDv1 fallback directive parsing and TTL data model

[joker23]

first set of changes to let client side fdv2 handle fdv1 fallback ttl. This PR adds logic to read FDv1 fallback values from both header and goodbye body.

---

Note

Low Risk
Additive types and parsing with tests; no orchestrator or runtime behavior changes in this diff.

Overview
Adds shared parsing for server-directed FDv1 fallback and how long to stay on FDv1 before retrying FDv2.

New fallbackDirective helpers read x-ld-fd-fallback / x-ld-fd-fallback-ttl (seconds → ms, case-insensitive "true", invalid TTL omitted, negatives clamped to indefinite) and protocolFallbackTTL on goodbye payloads for clients that cannot read stream headers (e.g. browser EventSource).

FDv2SourceResult now carries optional fdv1FallbackTtlMs on change-set and status results; factory helpers (changeSet, interrupted, terminalError, goodbye) accept the same TTL semantics (undefined = caller default, 0 = indefinite).

^{Reviewed by Cursor Bugbot for commit c902a80. Bugbot is set up for automated code reviews on this repo. Configure here.}

[github-actions]

@launchdarkly/js-sdk-common size report
This is the brotli compressed size of the ESM build.
Compressed size: 26365 bytes
Compressed size limit: 29000
Uncompressed size: 129044 bytes

[github-actions]

@launchdarkly/js-client-sdk-common size report
This is the brotli compressed size of the ESM build.
Compressed size: 38887 bytes
Compressed size limit: 39000
Uncompressed size: 212703 bytes

[github-actions]

@launchdarkly/browser size report
This is the brotli compressed size of the ESM build.
Compressed size: 179727 bytes
Compressed size limit: 200000
Uncompressed size: 831515 bytes

[github-actions]

@launchdarkly/js-client-sdk size report
This is the brotli compressed size of the ESM build.
Compressed size: 32006 bytes
Compressed size limit: 34000
Uncompressed size: 114336 bytes

[joker23]

@cursor review

[joker23]

@cursor review

[cursor]

✅ Bugbot reviewed your changes and found no new issues!

Comment @cursor review or bugbot run to trigger another review on this PR

^{Reviewed by Cursor Bugbot for commit b232737. Configure here.}

[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

View in Devin Review to see 1 additional finding.

[devin-ai-integration]

Devin Review found 1 new potential issue.

View 3 additional findings in Devin Review.