financial-benchmark: address Gemini review findings on services.xml schemas

F1 (Major): generateTestData mcpIdempotent corrected to true — pure
  deterministic function, same input always produces same output
F2 (Major): scenarioAnalysis assets array gains minItems:1, maxItems:2000;
  scenario_prices and probabilities gain minItems:2
F3 (Major): scenario_prices and probabilities descriptions now explicitly
  state they must have equal length (cross-property constraint)
F4 (Major): weights description corrected — states exact n_assets length
  required and clarifies normalize_weights=false means caller ensures sum=1
F5 (Major): normalize_weights description fixed — previously implied
  unconditional normalisation contradicting the boolean flag semantics
F6 (Minor): metadata mcpInputSchema gains additionalProperties:false since
  the operation accepts no body fields
F7 (Minor): monteCarlo mcpIdempotent changed to false — PRNG seeded from
  time() when random_seed absent, so unseeded calls are non-deterministic;
  comment documents when idempotency holds
F10 (Nit): scenarioAnalysis position_size type changed integer→number with
  description noting fractional positions unsupported by C implementation

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>