feat: add Cost Explorer (ce:*) service

[ShubhamDX]

Tracking issue for the Cost Explorer (ce:*) service — PR 2 of 3 from the scope discussion in #791.

Blocked by the AWS Pricing API service in #821 (cost synthesis consumes the Pricing snapshot).

Scope

AWS Cost Explorer API, JSON 1.1, X-Amz-Target: AWSInsightsIndexService.<Action> (signing name ce, endpoint prefix ce).

Operations:

• GetCostAndUsage — full support for TimePeriod, Granularity (DAILY / MONTHLY / HOURLY), Metrics (UnblendedCost, BlendedCost, AmortizedCost, NetUnblendedCost, NetAmortizedCost, UsageQuantity, NormalizedUsageAmount), GroupBy, and the recursive Filter expression tree (And / Or / Not / Dimensions / Tags / CostCategories).
• GetCostAndUsageWithResources — same shape, includes resource-level breakdown from Floci's existing resource state.
• GetDimensionValues — returns dimension values available in Floci's synthesized data.
• GetTags — returns tag keys/values across Floci resource state.
• GetReservationCoverage + GetReservationUtilization — stubs backed by static "no RI" responses unless an RI fixture is supplied.
• GetSavingsPlansCoverage + GetSavingsPlansUtilization — same stub approach.
• GetCostCategories + GetCostCategoryDefinition — minimal definitions list; empty if none configured.

Approach

Cost data is synthesized on each request from:

1. Floci's existing resource state (S3 buckets, EC2 instances, Lambda functions, …) at the requested TimePeriod.
2. The Pricing snapshot from feat(pricing): add AWS Price List Service support #821 (rate per (serviceCode, regionCode, usageType, …)).
3. The GroupBy / Filter expression applied after synthesis.

GroupBy=RECORD_TYPE must distinguish Usage, Credit, Tax, Refund, DiscountedUsage, SavingsPlanCoveredUsage, SavingsPlanNegation, SavingsPlanUpfrontFee, SavingsPlanRecurringFee — essential for testing any code path that separates gross usage from net billable outlay (credit-offset accounts, spend-limit checks, invoice-reconciliation code).

A single config knob for synthetic-credit injection (neutral name, matches Floci's existing FLOCI_SERVICES_<SVC>_* convention):

FLOCI_SERVICES_CE_CREDIT_USD_MONTHLY

emits a monthly Credit line item that offsets min(credit, usage) — gives downstream tools a deterministic way to exercise credit-offset logic without synthesizing their own fixtures.

Size estimate

~1000-1500 LOC Java + docs/services/ce.md.

Non-goals

• Anomaly detection (split into a separate PR; ce:GetAnomalies etc.).
• CUR emission (covered by PR 3, see separate tracking issue).
• Right-sizing recommendations (ce:GetRightsizingRecommendation) — out of scope for initial landing.

Open question for maintainers

The cost-synthesis path is a new shape in the repo (every other service either reads persisted state directly or returns a stub). Happy to go either way on:

1. A single concrete CostSynthesisBackend implementation with well-defined extension points, or
2. A formal SPI that downstream users can swap out entirely.

Option 1 keeps things minimal; option 2 is more future-proof if you anticipate users wanting non-trivial billing models (EDP discounts, custom Savings Plan math). Leaning toward option 1 unless you have a strong preference.

Reference

• Scope discussion: https://github.com/orgs/floci-io/discussions/791
• Blocking PR (Pricing): feat(pricing): add AWS Price List Service support #821

[ShubhamDX]

Scope tree for reviewer context, per #791:

• feat(pricing): add AWS Price List Service support #821 — AWS Pricing API (pricing:*) service — PR open (draft). Blocking prerequisite: this issue's cost synthesis consumes the Pricing snapshot shipped in feat(pricing): add AWS Price List Service support #821.
• This issue (CE) — unblocks feat: add CUR + BCM Data Exports services #823.
• feat: add CUR + BCM Data Exports services #823 — CUR + BCM Data Exports services — blocked by this issue.

[ShubhamDX]

PR up: #832