[ty] Propagate nested ParamSpec specializations

[dhruvmanila]

Summary

• defer preferred ParamSpec mappings until after argument inference
• propagate concrete inner-call ParamSpec specializations through return types to outer generic calls
• preserve the legacy solver's first-seen behavior for actual ParamSpec arguments

Root cause

The legacy specialization builder keeps the first mapping inferred for a ParamSpec. Bidirectional inference installed the symbolic mapping from the outer call's return context before processing the inner call's arguments, so the later concrete argument-derived mapping was discarded. Adding the contextual mapping last makes it a fallback while allowing the concrete specialization to flow through the inner return type into the outer call.

Validation

• exact reproduction from invalid-argument-type false positive: ParamSpec not propagated through composed higher-order functions ty#3691
• cargo test -p ty_python_semantic with snapshot-update environment variables
• cargo clippy --workspace --all-targets --all-features -- -D warnings
• uvx prek run --files crates/ty_python_semantic/src/types/call/bind.rs crates/ty_python_semantic/resources/mdtest/bidirectional.md crates/ty_python_semantic/resources/mdtest/generics/pep695/paramspec.md

Closes astral-sh/ty#3691

[astral-sh-bot]

Typing conformance results

No changes detected ✅

Current numbers

The percentage of diagnostics emitted that were expected errors held steady at 92.23%. The percentage of expected errors that received a diagnostic held steady at 87.42%. The number of fully passing files held steady at 92/134.

[astral-sh-bot]

Memory usage report

Memory usage unchanged ✅

[astral-sh-bot]

ecosystem-analyzer results

Lint rule	Added	Removed	Changed
invalid-argument-type	0	2	0
Total	0	2	0

Flaky changes detected. This PR summary excludes flaky changes; see the HTML report for details.

Raw diff:

openlibrary (https://github.com/internetarchive/openlibrary)
- openlibrary/core/bookshelves.py:174:53 error[invalid-argument-type] Argument to bound method `AsyncBridge.wrap` is incorrect: Expected `(**_P@staticmethod) -> Coroutine[Any, Any, None]`, found `def add_solr_works_async(readinglog_items, fields: Iterable[str] | None = None) -> CoroutineType[Any, Any, None]`

zulip (https://github.com/zulip/zulip)
- zerver/management/commands/runtornado.py:100:35 error[invalid-argument-type] Argument to function `sync_to_async` is incorrect: Expected `(**_P@push_async_callback) -> None`, found `def remove_signal_handlers() -> None`

Full report with detailed diff (timing results)