Enforce microVersionId conflicts at the metadata write level

[maeldonn]

The JS pre-checks in putMetadata are not atomic: two concurrent CRR writes can
both pass them and let a stale revision overwrite a newer one. This attaches a
conditional put ($gt on microVersionId) so the write itself rejects stale
revisions with PreconditionFailed, mapped to the same conflict response as the
pre-checks. Enabled only on backends supporting conditional puts (mongodb).

Issue: CLDSRV-952

[bert-e]

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[bert-e]

Waiting for approval

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.08%. Comparing base (0d08680) to head (9b5d259).
✅ All tests successful. No failed tests found.

Additional details and impacted files

Files with missing lines	Coverage Δ
lib/routes/routeBackbeat.js	79.04% <100.00%> (+0.30%)	⬆️

... and 2 files with indirect coverage changes

@@                 Coverage Diff                 @@
##           development/9.4    #6235      +/-   ##
===================================================
- Coverage            86.10%   86.08%   -0.02%     
===================================================
  Files                  212      212              
  Lines                14483    14490       +7     
===================================================
+ Hits                 12470    12474       +4     
- Misses                2013     2016       +3     

Flag	Coverage Δ
file-ft-tests	70.09% <42.85%> (-0.07%)	⬇️
file-ft-tests-null-compat	70.62% <42.85%> (-0.02%)	⬇️
kmip-ft-tests	27.92% <0.00%> (-0.02%)	⬇️
mongo-v0-ft-tests	71.19% <57.14%> (-0.04%)	⬇️
mongo-v1-ft-tests	71.25% <57.14%> (+0.04%)	⬆️
multiple-backend	36.14% <42.85%> (+<0.01%)	⬆️
s3c-ft-tests-v0	65.12% <42.85%> (-0.02%)	⬇️
s3c-ft-tests-v0-null-compat	65.18% <42.85%> (-0.02%)	⬇️
s3c-ft-tests-v1	65.10% <42.85%> (-0.02%)	⬇️
sur-tests	34.77% <0.00%> (-0.91%)	⬇️
sur-tests-inflights	37.52% <0.00%> (-0.05%)	⬇️
unit	73.12% <100.00%> (+0.11%)	⬆️
utapi-v2-tests	35.09% <0.00%> (-0.02%)	⬇️

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[claude]

Only asserting statusCode is 409 doesn't distinguish this from the StaleMicroVersionIdException 409 response. Assert the body code so a regression that returns the wrong 409 type fails the test.

Suggested change

	assert.strictEqual(mockResponse.statusCode, 409);
	assert.strictEqual(mockResponse.statusCode, 409);
	assert.strictEqual(mockResponse.body.code, 'MicroVersionIdAlreadyStoredException');

[claude]

Same here — assert the body code so this test doesn't pass if a different 409 path fires.

Suggested change

	assert.strictEqual(mockResponse.statusCode, 409);
	assert.strictEqual(mockResponse.statusCode, 409);
	assert.strictEqual(mockResponse.body.code, 'StaleMicroVersionIdException');