fix: paginate environment-document for local evaluation

[higordasneves]

Closes #205

---

Summary

Adds pagination support for the local-evaluation environment document, addressing Flagsmith/flagsmith#4664 on the
Go SDK side. Before this PR the Go client only consumed the first page, silently dropping identity overrides that fall on
subsequent pages.

What changed

• UpdateEnvironment now follows the Link header, accumulating IdentityOverrides across pages, then maps the merged EnvironmentModel into the
  engine evaluation context exactly once at the end of the refresh cycle.
• New helpers: ExtractNextPage (parses the page_id query param out of the Link header) and checkEnvironmentMemoryAlloc (per-refresh memory
  cap).
• Two new client options:
  • WithLocalEvaluationPageLimit(int) — caps pages per refresh. Default is 1, which preserves pre-PR behaviour for users that haven't opted
    in. 0 means unlimited.
  • WithLocalEvaluationMemoryAllocLimit(int) — caps the accumulated identity_overrides JSON size (in bytes) across pages. 0 means no limit.
• Fixtures: OverriddenIdentifierPage2, PageID, PageIDEncoded, EnvironmentJsonPage2, and a PaginatedEnvironmentDocumentHandler that serves a
  Link-paginated two-page response.

Behaviour notes

• Backwards compatible by default. localEvaluationPageLimit defaults to 1, so existing users see the exact same single-page fetch they get
  today. They opt in to pagination by setting the limit to 0 (unlimited) or a value > 1.
• First page is authoritative for everything except identity_overrides. Subsequent pages contribute only their IdentityOverrides;
  feature_states, project, segments, etc. are taken from page 1. This matches the Edge API's pagination contract.
• Memory limit is a soft stop. When appending the next page would exceed the byte limit, that page (and any pages after it) are dropped and a
  warning is logged; the refresh otherwise succeeds with what was loaded.

Manual tests

Backward compatibility test

Pagination test

[emyller]

Thanks for the contribution! This will help us immensely in moving other projects too.

I've left one blocking comment, and another potential one.

[emyller]

I've opened #205, including updated acceptance criteria, and updated the description of this PR pointing to it. If you decide to meet all A/C — you're not required to! —, please update the comment to "closes".

[higordasneves]

I've opened #205, including updated acceptance criteria, and updated the description of this PR pointing to it. If you decide to meet all A/C — you're not required to! —, please update the comment to "closes".

@emyller Pushed an update addressing the acceptance criteria from #205. Pagination now always follows Link: rel="next" to the end, the two opt-in limits are gone, and a warning is logged when a refresh exceeds WithEnvironmentRefreshInterval. I've also replied inline on both threads with the reasoning behind the original conservative approach for context.

[emyller]

Great work, thank you! Adding to our release pipeline soon.

[higordasneves]

@emyller ops, linter issue solved haha

[emyller]

Sorry, it looks like we need to handle tooling updates around linting/CI. Allow us a bit to get that sorted.

[emyller]

@higordasneves Can you please update this branch with latest main? The CI error above should be gone.

[higordasneves]

@emyller done

[emyller]

FYI this has been release as part of v5.1.0. Again, thanks so much for the contribution!