fix: resolve actor/context templates in through relationship filters

[barnabasJ]

Problem

Fixes #788.

When a has_many relationship carries a filter that uses a ^context/1 or ^actor/1 template, and that relationship is used inside a has_many ... through relationship, loading the through relationship fails:

** (RuntimeError) Unsupported expression in Elixir.AshPostgres.SqlImplementation query:
{:_context, :sample_context}

The intermediate relationship's filter was applied via Ash.Query.do_filter without filling the template, so the raw template ({:_context, :sample_context}) was handed to the SQL implementation, which cannot compile it. The same class of bug affected ^actor/1, ^arg/1 and ^tenant/0 templates.

Fix

Fill ^actor/^context/^arg/^tenant templates before the intermediate relationship filter is compiled into SQL, in both through code paths in lib/data_layer.ex (the recursive build_through_queries/4 used for 3+ hop paths, and the inline path used for 2-hop / many_to_many).

This mirrors how a directly loaded relationship filter is hydrated in Ash.Actions.Read.Relationships: actor/args/tenant come from the source query, while the context is taken from the query the filter is being applied to (after its set_context). As a result, loading a through relationship resolves templates identically to loading each hop directly.

Tests

Added intermediate relationships with ^context and ^actor filters on the Through.School test resource, plus through relationships using them, covering both the 3-hop (build_through_queries) and 2-hop (inline) paths. New tests in test/through_relationships_test.exs assert:

• the ^context/^actor templates no longer raise, and
• the ^actor template resolves to the actor's value and filters the through relationship correctly.

Full suite: 832 passing, mix format clean, mix ash_postgres.generate_migrations --check clean.

🤖 Generated with Claude Code

[zachdaniel]

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