fix: align session expiration behavior across API and SDK

[acsoto]

What type of PR is this?

/kind bug

What this PR does / why we need it:

Aligns the session expiration contract across the Router, Workload Manager, and Python SDK:

• returns SESSION_NOT_FOUND only when the stored session is missing;
• treats request ttl as a maximum lifetime capped by spec.maxSessionDuration, independent of the idle spec.sessionTimeout;
• raises SessionNotFoundError and clears stale session IDs without implicit recreation or retry.

Application-level 404 responses remain ordinary HTTP errors.

Which issue(s) this PR fixes:

Fixes #459

Special notes for your reviewer:

ttl is expressed in seconds. Omitting it preserves the workload CR/default lifetime.

Validated with Go unit tests and vet for the changed packages, 26 Python SDK tests, Ruff, and diff checks. Prepared with AI assistance and manually reviewed and validated.

Does this PR introduce a user-facing change?:

Session expiration now returns a structured error, Python SDKs invalidate stale session IDs, and request-level TTL is enforced within workload limits.

[volcano-sh-bot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by:
Once this PR has been reviewed and has the lgtm label, please assign kevin-wangzefeng, yaozengzeng for approval. For more information see the Kubernetes Code Review Process.

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Details

Needs approval from an approver in each of these files:

• docs/OWNERS
• pkg/OWNERS
• sdk-python/OWNERS

Approvers can indicate their approval by writing /approve in a comment
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[codecov-commenter]

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Codecov Report

❌ Patch coverage is 94.73684% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 60.13%. Comparing base (5903457) to head (d1fef2d).
⚠️ Report is 7 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/api/errors.go	75.00%	1 Missing and 1 partial ⚠️
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Additional details and impacted files

@@            Coverage Diff             @@
##             main     #460      +/-   ##
==========================================
+ Coverage   59.98%   60.13%   +0.14%     
==========================================
  Files          36       36              
  Lines        3589     3615      +26     
==========================================
+ Hits         2153     2174      +21     
- Misses       1216     1220       +4     
- Partials      220      221       +1     

Flag	Coverage Δ
unittests	60.13% <94.73%> (+0.14%)	⬆️

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[acsoto]

cc @RainbowMango