Send the client version in a FlexMeasures-Client-Version request header

[Flix6x]

Closes #212

Why

A FlexMeasures server can't tell which client version is calling it.
When the server changes a response in a way older clients misread, the only way to protect those clients is manual. The host sets FLEXMEASURES_LEGACY_JOB_RESPONSES_MAX_INCOMPATIBLE_CLIENT_VERSION and adds a version attribute to each affected asset.
A recent case: FlexMeasures ≥ v1.0.0 returns 202 while a scheduling job runs, and client 0.9.3 read that as the finished schedule. The Home Assistant integration then failed with KeyError: 'values' (see FlexMeasures/flexmeasures-ha-integration#31).

With the version in every request, the server can make that decision per request, with no attributes to set.

What

• Every request now sends FlexMeasures-Client-Version: <flexmeasures_client.__version__>. It's added in get_headers(), so it covers:
  • unauthenticated requests (requestAuthToken, get_versions), so the server knows the version from the first request;
  • authenticated API requests;
  • file uploads, which drop only Content-Type from those headers.
• The header name lives in constants.CLIENT_VERSION_HEADER.

Why a custom header rather than User-Agent

• It mirrors the server's FlexMeasures-Version response header.
• User-Agent belongs to the calling app. Home Assistant's shared session already sets it to HomeAssistant/… aiohttp/… Python/…. Overwriting that would hide the app from server logs, and merging our token in would mean editing a header the client doesn't own.
• A dedicated header holds just the version, so the server can compare versions without parsing the free-form User-Agent list.

Adding a flexmeasures-client/<version> token to User-Agent for logs and proxies could be a separate change.

Tests

• New test_every_request_sends_the_client_version: runs a token request, a versions request, an authenticated GET and a file upload, and checks that all four send the header.
• The 16 existing tests that compare full request headers now include the new header.
• Checked that the tests can fail: with the header removed from get_headers(), 17 tests fail, including the new one. With it restored, the whole non-S2 suite passes (224 tests).
• All pre-commit hooks pass (isort, black, flake8, mypy).

Follow-up on the FlexMeasures side (not in this PR)

• use_legacy_job_responses() could read this header before checking asset attributes.
• Clients up to 0.9.5 never send the header, so a missing header must mean "unknown", not "old".
• Optionally, store the last-seen client version on the user, to see which client versions are in use.

🤖 Generated with Claude Code

https://claude.ai/code/session_019heS4SVj8UqZ8BiMqDvoXj

[coveralls]

Coverage Report for CI Build 35134394770

Coverage increased (+0.007%) to 96.831%

Details

• Coverage increased (+0.007%) from the base build.
• Patch coverage: 4 of 4 lines across 2 files are fully covered (100%).
• No coverage regressions found.

Uncovered Changes

No uncovered changes found.

Coverage Regressions

No coverage regressions found.

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Coverage Stats

Relevant Lines:	852
Covered Lines:	825
Line Coverage:	96.83%
Coverage Strength:	9.68 hits per line

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💛 - Coveralls