fix(sessions): dedupe subagent IDs across disk paths

[rioyu123]

Summary

This pull request makes filesystem-backed list_subagents() return each logical agent ID once when the same ID exists under multiple physical subpaths.

• preserve the first discovery order while deduplicating IDs
• align the disk helper with list_subagents_from_store() and its TypeScript parity contract
• document the uniqueness guarantee
• add regression coverage for a direct transcript plus nested duplicate

Why

Subagent transcripts can exist both directly under subagents/ and under nested workflow directories. The disk helper previously returned duplicate IDs for those paths even though callers can only address a subagent by ID and the store-backed helper already deduplicates them.

The change is intentionally limited to list_subagents(): traversal, exception behavior, get_subagent_messages(), and public signatures remain unchanged.

Test plan

• pytest tests/test_sessions.py::TestListSubagents tests/test_session_helpers_store.py::TestSubagentsFromStore -q -p no:cacheprovider - 29 passed
• ruff format --check src/claude_agent_sdk/_internal/sessions.py tests/test_sessions.py
• ruff check src/claude_agent_sdk/_internal/sessions.py tests/test_sessions.py
• mypy src - clean
• pytest tests --ignore=tests/test_changelog.py -q -p no:cacheprovider - 1471 passed, 14 skipped
• Full Windows suite: 1472 passed, 14 skipped, with the five pre-existing locale-decoding failures in tests/test_changelog.py; those are unrelated to this patch and are addressed separately in fix(tests): read changelog as UTF-8 #1236

AI assistance disclosure

I used AI assistants for implementation review. I independently reproduced the bug, verified the regression test against the old behavior, reviewed the resulting diff, and ran the checks above.