Add baseBackoffMS support

[vbabanin]

Summary

This PR brings the driver's Client Backpressure implementation up to date with the latest
specification changes: the server-supplied baseBackoffMS override, the corrected
exponential backoff formula exponent, and the versioned backpressure: "2" handshake
flag. Also adds MongoDB 9.0 to the Evergreen version matrix so the new 9.0-gated
prose test has a CI variant to run against.

Tickets

JAVA	DRIVERS	Description
JAVA-6238	DRIVERS-3535	Client Backpressure with baseBackoffMS — backoff formula exponent, baseBackoffMS server override, handshake backpressure: "2"
JAVA-6255	DRIVERS-3578	Correct sum of backoffs for the convenient transactions API backoff prose test
JAVA-6265	DRIVERS-3584	Provide updated retryable overload error code examples for 9.0 — out of scope for this PR (documentation deliverable, tracked separately as TODO)

Upstream spec commits:

• d6fc118 (DRIVERS-3535)
• 615e0f9 (DRIVERS-3578)

Commits

Commit	Ticket	Summary
a8c755e829	JAVA-6238	use retry number as backoff exponent
407b3f0430	JAVA-6255	correct withTransaction backoff test sum to 2.3s
ac31095dc9	JAVA-6238	update overload backoff prose test to 0.6s sum per new formula
9b6d1a4995	JAVA-6238	send backpressure: "2" in handshake
64d1a438b1	JAVA-6238	support server-supplied baseBackoffMS override for overload backoff
d5377190cd	JAVA-6238	add baseBackoffMS override prose test (Test 5)
7a42e3ac5b	JAVA-6238	add MongoDB 9.0 to Evergreen version matrix
5472e7b055	JAVA-6238	extract baseBackoffMS field name to a constant

Changes

Backoff formula exponent (JAVA-6238 / DRIVERS-3535)

The spec changed the exponential backoff to use the retry number directly as the
exponent, instead of one less than the retry number:

• Overload retry: jitter * min(MAX_BACKOFF, BASE_BACKOFF * 2^attempt) (was 2^(attempt-1))
  → first retry 200ms, second 400ms (was 100ms / 200ms).
• Convenient transactions API: jitter * min(BACKOFF_INITIAL * 1.5^attempt, BACKOFF_MAX)
  (was 1.5^(attempt-1)).

Both share ExponentialBackoff.calculateBackoffMs, so a single-line exponent fix
covers both. BASE_BACKOFF remains 100ms; MAX_BACKOFF remains 10000ms.

Server-supplied baseBackoffMS override (JAVA-6238 / DRIVERS-3535)

When an overload error document contains a positive top-level baseBackoffMS field,
the driver now uses it in place of the 100ms BASE_BACKOFF constant:

• New ExponentialBackoff.calculateOverloadBackoff(int, @Nullable Long baseBackoffMs)
  overload. A null or non-positive value falls back to the 100ms default; a positive
  value replaces it. The 10000ms cap is preserved.
• SpecRetryPolicy extracts baseBackoffMS from MongoCommandException.getResponse()
  and passes it through. Extraction is lenient: absent, non-numeric, or non-positive
  values are ignored.

Handshake backpressure: "2" (JAVA-6238 / DRIVERS-3535)

The handshake now sends backpressure: "2" — the BSON string "2", not a boolean
and not the number 2 — to version the supported backpressure specification. Previously
the driver sent backpressure: true. The value is sent unconditionally on every hello
(no server-version gating), matching the Python and Node reference implementations;
the flag is currently unused by the server and older servers ignore unexpected
handshake field values.

Test updates

• Prose Test 1 (BackpressureProseTest.operationRetryUsesExponentialBackoff):
  expected overload backoff sum 0.3s → 0.6s.
• New prose Test 5 (BackpressureProseTest.overloadErrorsWithBaseBackoffMsOverrideBaseBackoff):
  verifies baseBackoffMS overrides the default backoff. Gated on
  serverVersionAtLeast(9, 0); pins jitter to 1, toggles
  externalClientBaseBackoffMS via setParameter, and asserts timing bounds
  (default run ≥ 600ms; override run ≥ 300ms and < 600ms).
• Handshake prose Test 9 (AbstractClientMetadataProseTest): asserts the
  backpressure field is the string "2".
• withTransaction backoff test (WithTransactionProseTest.testRetryBackoffIsEnforced):
  expected sum of 13 backoffs 1800ms → 2300ms.
• Unit tests (ExponentialBackoffTest): updated the transaction backoff expected
  array for the new exponent; added 4 tests covering the baseBackoffMS override
  (default-when-absent, positive-override, non-positive-ignored, max-cap-preserved).
• Groovy unit spec (InternalStreamConnectionInitializerSpecification): updated
  the 4 handshake expectations to backpressure: "2".

Evergreen (JAVA-6238)

Added a 9.0 entry to the version axis in .evergreen/.evg.yml and included 9.0
in all test matrix variants that currently test against 8.0, mirroring the Python
and Node drivers. This gives the 9.0-gated prose Test 5 a pinned CI variant. The
Atlas-specific MONGODB_VERSION: "8.0" deploy target is left untouched (Atlas
manages its own server version).