fix: parse day and fractional-second components in FromTimeSpan (cherry-pick of #435)

[NickJosevski]

Cherry-pick of #435 by @Scott-Emberson, re-hosted on a branch in this repo solely so CI can run. Authorship is preserved on the commit — this is a mechanical move, not a rewrite, and the work and the analysis are entirely Scott's.

Fork PRs cannot satisfy the required test check: integration-tests.yml needs secrets.DB_IMAGE_SA_PASSWORD, OD_IMAGE_ADMIN_API_KEY and OCTOPUS_SERVER_BASE64_LICENSE, and GitHub withholds secrets from pull_request runs originating on a fork, so the job dies at Initialize containers before any Go executes. #435 is red for that reason and no other.

The bug

FromTimeSpan read the day component from timeSpan[0:0] — always the empty string — so every value carrying days parsed to zero. Fixed offsets also assumed a single-digit day, fractional seconds were dropped, and an empty string panicked on a slice bound. It is duplicated verbatim in pkg/machines and pkg/machinepolicies; this fixes both.

Running main's parser against this one on the same inputs:

input	correct	main today	this PR
00:05:00	5m0s	5m0s	5m0s
01:30:00	1h30m0s	1h30m0s	1h30m0s
1.00:00:00	24h0m0s	0s	24h0m0s
7.12:30:00	180h30m0s	12h30m0s	180h30m0s
07.12:30:00	180h30m0s	0s	180h30m0s
10.00:00:00	240h0m0s	0s	240h0m0s
00:00:00.5000000	500ms	0s	500ms
1.02:03:04.5000000	26h3m4.5s	2h3m4s	26h3m4.5s
""	0s	panic	0s

Every day-bearing value was silently wrong rather than failing loudly. Plain hh:mm:ss is identical before and after, so nothing that works today regresses.

Blast radius

Read path only. ToTimeSpan, which produces what gets sent to the server, is untouched. All 14 in-repo callers of FromTimeSpan sit inside UnmarshalJSON on machine policy, cleanup policy and health check policy. It also closes a quiet data-loss path: today you can read a policy, write it back unchanged, and send a day-scale interval out as zero.

Two notes for the release

1. Negative time spans remain wrong, in both directions — pre-existing, not touched here, and arguably never supported:

   FromTimeSpan("-1.02:03:04") = -21h56m56s   // .NET means -26h3m4s
   FromTimeSpan("-00:05:00")   = 5m0s         // sign dropped entirely
   ToTimeSpan(-26h3m4s)        = "-1.-2:-3:-4"
   

2. This changes values consumers already read. Anyone getting 0s for these fields starts getting the true duration, which may surface as a one-off Terraform plan on machine policy timeouts. There is no CHANGELOG.md and goreleaser builds notes from commit subjects only, so this needs a note pasted onto the GitHub Release at tag time. Wording is in a comment on fix: parse day and fractional-second components in FromTimeSpan (changes parsed values) #435.

Once this merges

Close #435 pointing here, so Scott gets the credit and knows it wasn't rejected.