docs: remove oauth_pkce_example.py, which imports helpers that no longer exist

[LukasParke]

Summary

Fixes #583. examples/oauth_pkce_example.py imports oauth_create_sha256_code_challenge, oauth_create_authorization_url, and the matching request types from openrouter.utils. Those helpers were removed in e6b0242, so the example fails with ImportError. It was the only file in examples/ and nothing references it, so this PR deletes it instead of rewriting it against client.o_auth.

Not included: the README Resource Management snippet with empty with bodies (#589) and the os.getenv(..., "") default (#591). Both come from Speakeasy-generated README sections and usage snippets, so editing the README here would be overwritten on the next generation.

Searched existing PRs; none matched.

Link to Devin session: https://openrouter.devinenterprise.com/sessions/78072f08dc274fda9d8309045b1ab806
Open in Devin Desktop: https://openrouter.devinenterprise.com/desktop/session/78072f08dc274fda9d8309045b1ab806?variant=devin
Requested by: @LukasParke

[devin-ai-integration]

I'll fix CI failures and address comments from users with write access that start with 'Devin'.

• Disable automatic comment, CI, and merge conflict monitoring

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Original prompt from Luke

SYSTEM:
<latest_message>
Luke Parke (U0BF2BKT52N) [ts=1791235775.813579]: @Devin audit all open issues on our public Typescript, Python, and Go SDKs, and the terraform Provider repo, and list out the unique problems, and vet them to determine validity and severity
</latest_message>

=== BEGIN THREAD HISTORY (in #agents) ===
Luke Parke (U0BF2BKT52N) [ts=1791235775.813579]: @Devin audit all open issues on our public Typescript, Python, and Go SDKs, and the terraform Provider repo, and list out the unique problems, and vet them to determine validity and severity
=== END THREAD HISTORY ===
Channel ID: C07UF9XLTFF
Thread URL: https://openrouter.slack.com/archives/C07UF9XLTFF/p1791235775813579?thread_ts=1791235775.813579&amp;cid=C07UF9XLTFF

The <latest_message> is the message that you should use to guide your goals + task for this session, and you should use the rest of the slack thread as context.
A [ts=...] marker on a Slack message is that message's timestamp. To act on a specific message with the slack tool (e.g. adding an emoji reaction via the reaction command), pass that value as timestamp along with the Channel ID — no extra lookup call is needed.