fix: reconstruct current_path when base template defers before blocks

[jasmith-hs]

Description

PR description authored by Claude Code.

Base templates that extends and set a deferred variable before their block
definitions were losing the current_path reference across eager passes. The eager
first pass emitted output without {% set current_path = '...' %}, so the second pass
had no base-template path to resolve relative includes against, breaking path-dependent
tags in any such base template.

The path setter was only written when resolveBlockStubs() added deferred tokens
(deferred content inside block bodies), not when the base template itself deferred
tokens before its blocks.

This PR contains two commits:

• reconstruct current_path when base template defers before blocks — widens the
  path-setter condition so it also fires when the base template defers before its blocks.
• only reconstruct current_path when an extends actually deferred — fixes a
  regression from the first commit. preserveBlocks was unsound outside the extends
  loop: numDeferredTokensBefore starts at 0 and is only advanced inside the
  while (!extendParentRoots.isEmpty()) loop, so a template with no extend parents
  never entered the loop and preserveBlocks collapsed to deferredTokens.size() > 0.
  Any deferred template (for, import, set, include, ...) then injected a spurious
  {% set current_path = null %}. The fix gates preserveBlocks on an extends having
  actually been processed, so the path setter only fires in the intended
  base-template-defers-before-blocks case.

BRAVE

Backwards Compatibility

• The no-extends path now falls back to the original resolveBlockStubs growth check, restoring pre-first-commit behavior.
• Only the extends + defer-before-blocks case changes: it now correctly emits the current_path setter.

Rollout and Rollback Plan

• Library change; rolls out with the next jinjava release. Revert the PR to roll back.

Automated Testing

• Added itDefersSetInBaseBeforeBlock / itDefersSetInBaseBeforeBlockSecondPass covering the base-defers-before-block case.
• Full suite green: 2130 run, 0 failures (was 279 failures after the first commit alone).

Verification

• Confirmed the spurious {% set current_path = null %} no longer appears in the previously-failing eager tag tests (EagerForTag, EagerImportTag, EagerSetTag, EagerIncludeTag, EagerFromTag, etc.).

Expect Dependencies to Fail

• None expected.

REVIEWERS: Please review both the code changes and the answers above, and validate that they match the expectations for BRAVE