perf: eliminate redundant map lookups in swssPrioNotify and swssOutputNotify

[xq9mend]

Why

swssPrioNotify and swssOutputNotify each perform two map lookups on the success path:

1. find() to check if the key exists
2. at() to retrieve the value

Both are O(log n) on std::map, so the success path pays the cost twice unnecessarily.

How

Store the iterator returned by find() and use it->second directly in the else branch, eliminating the second lookup.

How to verify

Existing logger unit tests pass. Behavior is identical — only the number of map traversals changes.

[mssonicbld]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[xq9mend]

/azpw run Azure.sonic-swss-common

[mssonicbld]

⚠️ Notice: /azpw run only runs failed jobs now. If you want to trigger a whole pipline run, please rebase your branch or close and reopen the PR.
💡 Tip: You can also use /azpw retry to retry failed jobs directly.

Retrying failed(or canceled) jobs...

[mssonicbld]

/azp run

[mssonicbld]

No Azure DevOps builds found for #1214.

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[mssonicbld]

Hi, there are workflow run(s) waiting for approval, you may be first-time contributor. I will notify maintainers to help approve once PR is approved. Thanks!

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[xq9mend]

/azpw run Azure.sonic-swss-common

[mssonicbld]

⚠️ Notice: /azpw run only runs failed jobs now. If you want to trigger a whole pipline run, please rebase your branch or close and reopen the PR.
💡 Tip: You can also use /azpw retry to retry failed jobs directly.

Retrying failed(or canceled) jobs...

[mssonicbld]

Retrying failed(or canceled) stages in build 1144010:

✅Stage Test:

• Job vstest: retried.

[xq9mend]

/azpw run Azure.sonic-swss-common

[mssonicbld]

⚠️ Notice: /azpw run only runs failed jobs now. If you want to trigger a whole pipline run, please rebase your branch or close and reopen the PR.
💡 Tip: You can also use /azpw retry to retry failed jobs directly.

Retrying failed(or canceled) jobs...

[mssonicbld]

Retrying failed(or canceled) stages in build 1144010:

✅Stage Test:

• Job vstest: retried.

[mssonicbld]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[xq9mend]

The Test vstest failure is a systemic CI issue unrelated to this PR. The p4rt test group in sonic-swss/tests/p4rt/ is failing across all sonic-swss-common PRs (exit code 123, zero tests passing) due to a mismatch between the latest sonic-swss master test code and the pre-built docker-sonic-vs image. This affects every open sonic-swss-common PR right now — see #1211, #1212, #1213, #1214 all showing the same failure.

The code change in this PR is correct and unrelated to p4rt.