[app-server] Add stored-thread view helpers

[adamvy-oai]

Overview

This is the third change in a five-PR stack that improves the Rust embedding API for codex-app-server. It gives stored-thread consumers one intentional module for producing the same views, filters, and pages as the built-in request processors.

What

• add codex_app_server::thread_views
• expose from_stored_thread() for projecting a StoredThread into the app-server representation
• expose ThreadSourceFilter for the two-stage source filtering used by thread list and search requests
• clarify that an empty backing-store allowed_sources list applies no source filter
• expose paginate_loaded_thread_ids() and paginate_turns()
• move the built-in request processors to these helpers without intentionally changing request behavior

Why

Stored-thread projection and pagination previously lived inside request-processing code, while source filtering lived in a separate private module. Embedders with another ThreadStore would have to reproduce that behavior to return app-server-compatible views.

The new module defines that behavior once and makes the built-in request processors use the same API. ThreadSourceFilter retains the existing split between coarse store filtering and precise in-memory classification for source kinds that the store cannot distinguish directly. The backing-store contract is now explicit: an empty allowed_sources list applies no store-level source filter, so ThreadSourceFilter::matches() can perform the precise classification afterwards.

Stack

Each PR is based on the preceding branch:

1. #28459 — custom in-process thread stores
2. #28460 — lossless in-process event delivery
3. #28461 — stored-thread view helpers (this PR)
4. #28462 — rollout-history turn boundaries
5. #30369 — durable external thread goals

Validation

Validation for the final stacked commits:

• six focused thread_views tests, including unfiltered backing-store queries followed by precise source classification
• loaded-thread pagination integration test
• turn pagination and item-view integration tests
• just test -p codex-app-server: 886 passed; one timing retry passed on its second attempt
• just test -p codex-core: 2,717 passed; eight timing-sensitive failures passed in focused reruns, while five high-output code-mode failures remained
• just test --test-threads=8: 10,944 of 10,958 passed; the remaining failures were the same five high-output tests, three timing-sensitive tests that pass in focused reruns, and six tests in untouched exec/TUI code affected by this host's managed permission policy
• a representative high-output failure reproduces on the exact upstream/main commit
• just fix -p codex-app-server, just fix -p codex-core, and just fmt; final worktree clean

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