Query-chain relation-collection detection: nullsafe (?->) and $this-rooted chains

[mikebronner]

Summary

Implements #272 — extends the query-chain executed-relation-collection detection (issue #246's shape) to nullsafe (?->) and $this-rooted chains, and pins the root-scope-before-relation composition with a dedicated end-to-end test.

Changes

• flow.rs — resolve_collection_relation now accepts nullsafe_member_call_expression links as chain structure (mixed ->/?-> chains included), resolves a bare-$this root via the enclosing class's FQCN, and handles a $this->prop root by making the property the relation claim (the deepest call rides the heuristic-hop queue). Returns a new CollectionRelation struct carrying a from_call flag, since the property claim must seed RelationHopKind::Claim (miss → clear) rather than CallClaim (miss → consult scopes).
• extractor.rs — the bare-$var receiver path passes the struct's from_call through instead of hardcoding true.
• member_resolver.rs — enclosing_class_fqcn promoted to pub(crate) for reuse by the flow walk.
• flow/tests.rs — 8 new unit tests (nullsafe, mixed-operator, nullsafe mode-flip gate, $this call root, $this->prop root, $this outside a class, $this builder-call gate, non-$this property root); existing helper projects the new struct so all pre-existing assertions run unchanged.
• diagnostics/tests.rs — 8 new end-to-end tests: paired positive/typo for the nullsafe, $this-rooted, and $this->prop-rooted shapes, plus the paired scope-before-relation pin.

Acceptance Criteria

• $var = $user?->competitions()?->get() (nullsafe) detected as an executed-relation collection; downstream chain validates against the related table — paired positive/typo tests (nullsafe_relation_collection_*).
• $var = $this->competitions()->get() and $var = $this->user->competitions()->get() detected likewise, relation resolved against the enclosing class's model — paired positive/typo tests (this_rooted_*, this_property_rooted_*).
• Existing bare-$var, static-root, (new X)-root detection and the RelationProperty property-access path do not regress — full suite passes unchanged (2790 tests).
• Dedicated end-to-end regression test for the root-scope-before-relation shape, mirroring scoped_relation_collection_flags_typo_on_related_table with a paired positive/typo pair (root_scope_before_relation_collection_*).

Test Plan

• All existing tests pass (cargo test: 2241 + 469 + 80, 0 failures)
• New tests cover the changes (16 added: 8 unit, 8 end-to-end)
• cargo clippy --all-targets clean, cargo fmt applied

Fixes #272

[mr-sherlock-holmes]

✅ Approved

Review Summary

• Extends query-chain executed-relation-collection detection (issue #246's shape) to nullsafe (?->) chains and $this-rooted / $this->prop-rooted chains, plus a dedicated end-to-end pin for the root-scope-before-relation composition.
• Every acceptance criterion is met:
  • AC #1 (nullsafe): is_member_call (flow.rs) matches both member_call_expression and nullsafe_member_call_expression, so $user?->competitions()?->get() walks identically to the plain--> form. Paired nullsafe_relation_collection_validates_against_related_table / _flags_typo_on_related_table assert against the related table. ✅
  • AC #2 ($this-rooted): the root-kind match resolves bare-$this via enclosing_class_fqcn (call claim, from_call: true) and $this->prop via the property claim (from_call: false); paired positive/typo tests for both this_rooted_* and this_property_rooted_*. ✅
  • AC #3 (no regression): member_access_receiver / RelationProperty path is byte-identical to main; the pre-existing shapes still return from_call: true. Full suite passes unchanged (2790 tests). ✅
  • AC #4 (scope-before-relation pin): root_scope_before_relation_collection_* uses $user->forCurrentTenant()->competitions()->get() and genuinely mirrors scoped_relation_collection_flags_typo_on_related_table, adding the positive counterpart the mirrored test lacked. ✅
• Tests verified meaningful, not tautological: each typo test would legitimately produce zero diagnostics (not one) without the fix, because pre-fix resolve_collection_relation/classify_rhs strictly matched only member_call_expression and had no $this special-case — so the assert_eq!(diags.len(), 1) genuinely fails without the change. Message assertions substitute the resolved table name, so they can only pass if detection actually rerouted the type.
• The Claim vs CallClaim asymmetry is correct-by-design: the $this->prop property claim seeds RelationHopKind::Claim (miss → clear effective_model, no false positive), while call claims seed CallClaim (miss → consult local scopes). Each branch's is_known_builder_method gating matches its miss-semantics — the property branch safely omits the gate the call branch requires. Documented in the enum and the PR body.
• Security: clean — iterative chain walk (not recursion), no new unwraps/panics/unsafe, enclosing_class_fqcn → pub(crate) is a benign in-crate visibility promotion.
• CI: all checks green (LSP test/fmt/clippy, extension wasm/fmt/clippy).

Nicely scoped work: the new CollectionRelation struct threads the from_call distinction cleanly instead of hardcoding it, the doc-comments carry the reasoning, and the 16 tests (8 unit + 8 e2e) pin every new shape with paired positive/negative coverage plus the deliberate negative-shape gates ($this outside a class, $this builder-call, non-$this property root).

📋 Non-blocking follow-ups

• None.

Ready for @mikebronner to merge.