[Bugfix] Stream available for write / Arduino memory release

[h2zero]

Fixes #432
Fixes #433

Summary by CodeRabbit

• Bug Fixes
  • Improved compatibility with supported ESP32 Arduino environments.
  • Updated stream buffer availability reporting to use the standard integer format.
  • Safely reports zero available write space when no transmission buffer is present.
  • Improved consistency for applications checking whether data can be written to a Bluetooth stream.

[coderabbitai]

📝 Walkthrough

Walkthrough

Arrr, NimBLEStream::availableForWrite() now matches the base stream interface with a non-const int return type. ESP Arduino builds conditionally include esp32-hal-bt-mem.h when available.

Changes

Stream and ESP32 compatibility

Layer / File(s)	Summary
Stream write availability override src/NimBLEStream.h, src/NimBLEStream.cpp	The non-Arduino Stream stub now defines availableForWrite(). NimBLEStream overrides it with a non-const int return type and casts the TX buffer space.
Optional ESP32 Bluetooth memory include src/NimBLEDevice.cpp	When the header exists under ESP Arduino builds, the code includes esp32-hal-bt-mem.h.

Estimated code review effort: 2 (Simple) | ~10 minutes

Possibly related PRs

• h2zero/esp-nimble-cpp#384: This PR modifies the NimBLEStream::availableForWrite() API introduced there.
• h2zero/esp-nimble-cpp#412: This PR also modifies the non-Arduino Stream fallback and conditional header handling.

Poem

Arrr, the stream reports space anew,
With int returned and const cast away.
The ESP header joins the crew,
When the header is available.
Four small changes sail today.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	The changes address issue #432 with the Arduino Bluetooth memory header and issue #433 with the matching availableForWrite() signature.
Out of Scope Changes check	✅ Passed	All changes are limited to NimBLE Arduino integration and NimBLEStream API compatibility required by issues #432 and #433.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly identifies both main fixes: Stream write availability and Arduino memory release.

✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch bugfix/stream-availableForWrite-signature

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