fix(native): Replace unsafe atol() with folly::to<> in PrestoExchangeSource

[amitkdutta]

Summary:
Replace atol() calls with folly::to<> when parsing HTTP response headers
in PrestoExchangeSource. atol() silently returns 0 on empty/malformed
input and has undefined behavior on overflow. folly::to<> throws on invalid
input, making failures explicit rather than silently degrading.

Two call sites fixed:

• Content-Length header parsing (line ~301): guarded with an empty check since
  a missing Content-Length header is valid and should default to 0.
• Next page token header parsing (line ~334): no empty guard needed since the
  caller already checks for non-empty.

== NO RELEASE NOTE ==

Summary by Sourcery

Bug Fixes:

• Prevent silent failures and undefined behavior when parsing Content-Length and next page token HTTP headers by using folly::to instead of atol.

[sourcery-ai]

Reviewer's guide (collapsed on small PRs)

Reviewer's Guide

Replaces unsafe uses of C’s atol() in PrestoExchangeSource HTTP header parsing with folly::to<>, adding an explicit empty-header guard for Content-Length while relying on existing non-empty checks for the next-page token header.

File-Level Changes

Change	Details	Files
Make Content-Length parsing safe and explicit about missing/invalid values.	Include folly/Conv.h to use folly::to conversions. Store Content-Length header value in a local string variable for reuse and clarity. Guard Content-Length parsing so an empty header maps explicitly to 0 instead of invoking conversion. Use folly::to<uint64_t> to parse the Content-Length string, which throws on invalid or overflowing input.	presto-native-execution/presto_cpp/main/PrestoExchangeSource.cpp
Make next page token parsing use folly::to for safer numeric conversion.	Replace atol() with folly::to<int64_t> when converting the next page token header to an integer. Rely on the existing caller-side non-empty check, so no additional empty-guard logic is added around the conversion.	presto-native-execution/presto_cpp/main/PrestoExchangeSource.cpp

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[sourcery-ai]

Hey - I've left some high level feedback:

• Now that folly::to will throw on malformed Content-Length or next-token headers, consider wrapping these conversions in a small helper that catches folly::ConversionError and rethrows with additional context (e.g., URL, header name, value) to make diagnosing bad upstream responses easier.
• For contentLength, you special-case the empty string to 0; if there is any possibility of whitespace-only or otherwise non-canonical values, it may be safer to trim the header value before checking emptiness to avoid unexpected exceptions from folly::to.

Prompt for AI Agents

Please address the comments from this code review:

## Overall Comments
- Now that `folly::to` will throw on malformed `Content-Length` or next-token headers, consider wrapping these conversions in a small helper that catches `folly::ConversionError` and rethrows with additional context (e.g., URL, header name, value) to make diagnosing bad upstream responses easier.
- For `contentLength`, you special-case the empty string to 0; if there is any possibility of whitespace-only or otherwise non-canonical values, it may be safer to trim the header value before checking emptiness to avoid unexpected exceptions from `folly::to`.

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