fix(rest): log 4xx client exceptions at WARN instead of ERROR

[gaurav0107]

Summary

ApplicationExceptionMapper in the rest module logged every exception at
ERROR via logException, including exceptions that map to 4xx client
responses — NotFoundException (404), ConflictException (409),
IllegalArgumentException (400), AccessForbiddenException (403),
NoResourceFoundException (404). These are expected, recoverable client-side
conditions that Conductor already handled correctly, so logging them at
ERROR pollutes server error logs and makes genuine 5xx server-side failures
harder to spot. For example, every attempt to create an already-existing
resource emits an ERROR entry even though the handled result is a 409.

This change computes the mapped HttpStatus before logging and selects the
log level from the status class:

• 4xx client errors are logged at WARN
• 5xx, and any unmapped exception (which falls back to 500), remain at ERROR

The status mapping, response body, and the Monitors.error(...) metric are
unchanged; only the log level for 4xx responses changes. The log message
template and argument order are identical in both branches, so existing log
parsing is unaffected.

Closes #1045

Tests

Extended ApplicationExceptionMapperTest (JUnit 4 + Mockito + MockMvc):

• testException (generic Exception -> 500) is unchanged and still asserts
  ERROR-level logging.
• testClientErrorLoggedAtWarn throws ConflictException (-> 409) and asserts
  the mapper returns 409 and logs at WARN, never ERROR.

logger is a shared static mock in this test class, so @Before now clears
its invocation history to keep the two tests order-independent.

The change was written against the source and hand-formatted to match the
module's existing googleJavaFormat().aosp() style; CI validates the build,
tests, and Spotless formatting.