[fix] Preceding/following axes miss nodes when predicates add context tracking

[joewiz]

Closes #4109.

Problem

The preceding:: and following:: axes skip nodes when the input node set was produced by an expression with a context-dependent predicate. For example:

(: Expected: w1:pb1, w2:pb1, w3:pb1, w4:pb2, w5:pb2 :)
(: Actual:   w1:pb1, w2:PRECEDING_PB_NOT_FOUND, w3:PRECEDING_PB_NOT_FOUND, w4:pb2, w5:PRECEDING_PB_NOT_FOUND :)
for $w in doc("/db/test/test.xml")//w[exists(.)]
let $preceding-page := $w/preceding::pb[1]
return $w/@id || ":" || ($preceding-page/@id, "PRECEDING_PB_NOT_FOUND")[1]

The predicate [exists(.)] is a no-op logically, but it triggers context tracking: the self:: axis evaluation inside the predicate attaches a context entry to each NodeProxy in the result set. When these nodes are later used as input to preceding:: or following::, the context entries leak through copyContext() into the axis result nodes. The context ID deduplication check in NewArrayNodeSet.selectPreceding() / selectFollowing() then sees a matching context ID and incorrectly skips valid result nodes, treating them as duplicates.

Approach

The fix adds a NO_CONTEXT_ID guard to the existing IGNORE_CONTEXT check in both selectFollowing() and selectPreceding() of NewArrayNodeSet. When contextId is NO_CONTEXT_ID (meaning the axis step is not inside a predicate context tracking scope), the context-based deduplication check is skipped — the same behavior as IGNORE_CONTEXT.

The change is two lines (one per method), following the same pattern already used elsewhere in the codebase where IGNORE_CONTEXT and NO_CONTEXT_ID are treated as equivalent for dedup purposes.

XQTS results

Test set	Before (develop)	After (this PR)	Notes
prod-AxisStep	46 failures	46 failures	No change — all pre-existing
prod-AxisStep.following	2 failures	2 failures	No change — all pre-existing
prod-AxisStep.preceding	2 failures	2 failures	No change — all pre-existing

No regressions introduced.

Note: The XQTS axis tests do not cover the specific scenario of context tracking leaking through predicates on the input expression, which is why there is no XQTS improvement. The issue manifests specifically with persistent (database-stored) nodes and context-dependent predicates on the input node set — a scenario tested by the new XQSuite tests below.

Test plan

• 4 new XQSuite tests in axes-persistent-nodes.xqm covering both preceding:: and following:: axes with context predicates, in both FLWOR and simple map forms
• Full eXist test suite: 6,476 tests, 0 failures, 129 skipped
• XQTS prod-AxisStep, prod-AxisStep.following, prod-AxisStep.preceding: no regressions

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