Go SDK: Fix the mixed language task handler binding semantic

[jason810496]

• related: ADR-0006, lang-SDK ADR-0007
• related: Java SDK: Honor TaskFlow arg bindings sent by the supervisor #71188 (Java SDK), and the TS SDK side in a sibling PR

Why

Sole-struct binding fails the task when an argument no field claims arrives, but silently zero-fills a field no argument supplies. Nothing checks the stub signature against the Go one, so both are the same kind of mismatch, and neither is worth failing a run over when fields bind by name.

How

Fields bind by name like keyword arguments, so neither direction shifts anything the handler reads. Both are now logged before the task runs, and the task runs. A call can be wrong both ways at once, so each direction is its own message:

• Dag's call passed argument(s) the task handler does not declare
• Task handler declares argument(s) the Dag's call did not pass

The same two strings are used by every lang SDK, so one query finds them across languages. Captured defaults are not reported, as before.

type ReportInput struct {
    Region    string `arg:"region_code"`
    Threshold float64
}

report(region_code=region)  # no threshold
# Threshold stays 0.0, warned: Task handler declares argument(s) the Dag's call did not pass

report(region_code=region, threshold=0.75, extra="x")
# before: task fails on the unclaimed "extra"
# now:    warned, task runs

Unchanged: flat positional bindings still fail either way, because a dropped or added positional argument shifts every later one.

A spec that arrives empty is treated as every field unfilled, since that is what an argless call like my_task() sends, so it is warned about and the task runs.

What

• pkg/binding: the rules above, plus a log-capturing test helper.
• adr/0006 and README.md: both stated the old behavior. The README is what go.rst points users to for the full binding rules.
• airflow-core/adr/lang-sdk/0007: states the rule for every SDK, since the spec describes the stub signature and nothing checks it against the foreign handler's. Positional fails either way, named logs either way, and how much each runtime can see is a language difference rather than a rule difference.
• example/bundle/taskflowbinding: two new example tasks, one per direction, both asserted end to end in airflow-e2e-tests. via_struct_more_args is passed an argument its struct does not declare; via_struct_fewer_args declares a field the stub has no parameter for and keeps its Go zero value. ViaStructUnmatchedArg is also renamed ViaStructDefaultArg, since it now only demonstrates a captured default no field claims.

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Was generative AI tooling used to co-author this PR?

• Yes, with help of Claude Code Opus 5 following the guidelines

[FrankYang0529]

Overall LGTM. Thanks for the fix.