Add network subgraph data staleness protection

[neithanmo]

Problem

The Gateway routes queries to different indexers that may be synced to different blocks. When an indexer is significantly behind (sometimes months or years), it returns data that is correct for its block but stale from the client's perspective. This causes allocation counts to fluctuate wildly:

17:39:03  allocations: 502
17:40:02  allocations: 0    ← routed to 1.5-year-old indexer
17:43:03  allocations: 110  ← routed to months-old indexer
17:44:03  allocations: 512

Solution

Add a configurable staleness check that validates the _meta.block.timestamp from network subgraph responses. Data is accepted only if:

• It's fresh (within the configured threshold), OR
• It's stale but fresher than the current best data (progressive improvement)

This ensures:

• Service always starts (any data is accepted on init)
• Fresh data always wins
• Stale data can't replace better data
• Gradual improvement toward fresh data when all indexers are behind

Configuration

[subgraphs.network]
# Maximum allowed age of data in minutes. Set to 0 to disable.
# Default: 30
max_data_staleness_mins = 30

Testing

Added unit tests covering:

• Fresh data acceptance
• Stale data rejection (when not an improvement)
• Stale data acceptance (when fresher than current)
• Init behavior (accepts any valid data)
• Edge cases (missing/invalid timestamps)

[coveralls]

Pull Request Test Coverage Report for Build 21525823621

Details

• 112 of 171 (65.5%) changed or added relevant lines in 4 files are covered.
• 3 unchanged lines in 2 files lost coverage.
• Overall coverage increased (+0.03%) to 68.484%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
crates/service/src/service/router.rs	0	1	0.0%
crates/config/src/config.rs	0	2	0.0%
crates/tap-agent/src/agent.rs	0	2	0.0%
crates/monitor/src/allocations.rs	112	166	67.47%

Files with Coverage Reduction	New Missed Lines	%
crates/config/src/config.rs	1	79.67%
crates/monitor/src/allocations.rs	2	46.09%

Totals
Change from base Build 21485319913:	0.03%
Covered Lines:	10500
Relevant Lines:	15332

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💛 - Coveralls

[suchapalaver]

Suggested change

	/// Use `validate_freshness()` to check if the response is fresh enough.
	/// Use `check_and_update_freshness()` to check if the response is fresh enough.

Right?