libsql-sqlite3: Optimize libsql_pager_has_codec() function

[penberg]

Cache the result of libsql_pager_has_codec() in Pager struct to avoid walking through all VFSes every time a page is read.

[Copilot]

Pull Request Overview

This PR optimizes database encryption codec checks by caching the result during pager initialization instead of performing expensive VFS stack traversals on every page read operation.

Key changes:

• Added a hasCodec flag to the Pager struct to cache the codec state
• Replaced runtime calls to libsql_pager_has_codec() with direct checks of the cached hasCodec field
• Initialize the cache once during sqlite3PagerOpen() with an explanatory comment about the optimization

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 6 comments.

File	Description
libsql-sqlite3/src/pager.c	Added hasCodec field to Pager struct, cached codec check result during initialization, replaced function calls with field access
libsql-ffi/bundled/src/sqlite3.c	Same changes as pager.c in bundled SQLite amalgamation
libsql-ffi/bundled/SQLite3MultipleCiphers/src/sqlite3.c	Same changes as pager.c in SQLite3MultipleCiphers amalgamation

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